Low-Frequency Carbon Recombination Lines

The low-frequency carbon recombination lines (with wavelengths up to twenty five and even more meters) became important means of the low-density interstellar plasma diagnostics. An impressive amount of astrophysical information was obtained over the past twenty years the carbon lines were detecte...

Повний опис

Збережено в:

Бібліографічні деталі
Дата:	2001
Автори:	Konovalenko, A.A., Stepkin, S.V., Shalunov, D.V.
Формат:	Стаття
Мова:	Russian
Опубліковано:	Радіоастрономічний інститут НАН України 2001
Назва видання:	Радиофизика и радиоастрономия
Онлайн доступ:	http://dspace.nbuv.gov.ua/handle/123456789/122222
Теги:	Додати тег Немає тегів, Будьте першим, хто поставить тег для цього запису!
Назва журналу:	Digital Library of Periodicals of National Academy of Sciences of Ukraine
Цитувати:	Low-Frequency Carbon Recombination Lines / A.A. Konovalenko, S.V. Stepkin, D.V. Shalunov // Радиофизика и радиоастрономия. — 2001. — Т. 6, № 1. — С. 21-31. — Бібліогр.: 38 назв. — англ.

Репозитарії

Digital Library of Periodicals of National Academy of Sciences of Ukraine

id	irk-123456789-122222
record_format	dspace
spelling	irk-123456789-1222222017-07-01T03:03:11Z Low-Frequency Carbon Recombination Lines Konovalenko, A.A. Stepkin, S.V. Shalunov, D.V. The low-frequency carbon recombination lines (with wavelengths up to twenty five and even more meters) became important means of the low-density interstellar plasma diagnostics. An impressive amount of astrophysical information was obtained over the past twenty years the carbon lines were detected in the frequency range from 12 to 1400 MHz. Correspondingly, the maximum principal quantum numbers of the observed interstellar atoms are more than 800. The medium in the direction of Cassiopeia A is the best studied object, which forms such features. The volume of data for other galactic sources is also increasing. The observation of lines from such highly excited atoms is a unique and effective method for probing the physical conditions of the low-density interstellar plasma as well as the important means for the study of the physics of the high Ridberg state atoms. Низкочастотные рекомбинационные линии углерода (длины волн до 25 метров и более) стали удобным средством диагностики разреженной межзвездной плазмы. В течение последних 20 лет накоплен большой объем астрофизической информации. Линии углерода удалось обнаружить в диапазоне частот от 12 до 1400 МГц, что соответствует главным квантовым числам межзвездных атомов, превышающим 800. Наиболее полная информация получена для среды в направлении радиоисточника Кассиопея А. Постоянно увеличивается объем данных для других объектов Галактики. Наблюдение спектральных линий столь сильно возбужденных атомов оказалось уникальным и эффективным методом исследования физических свойств разреженной межзвездной плазмы, а также важным средством изучения физики ридберговских атомов. Низькочастотні рекомбінаційні лінії вуглецю (довжини хвиль до 25 метрів та більше) стали зручним засобом діагностики розрідженої міжзоряної плазми. Протягом останніх 20 років накопичено великий обсяг астрофізичної інформації. Лінії вуглецю вдалося виявити у діапазоні частот від 12 до 1400 МГц, що відповідає головним квантовим числам міжзоряних атомів, що перевищують 800. Найповнішу інформацію отримано для середовища у напрямку радіоджерела Кассіопея А. Постійно збільшується обсяг даних для інших об’єктів Галактики. Спостереження спектральних ліній високозбуджених атомів виявилося унікальним та ефективним методом дослідження фізичних властивостей розрідженої міжзоряної плазми, а також важливим засобом вивчення фізики рідбергівських атомів. 2001 Article Low-Frequency Carbon Recombination Lines / A.A. Konovalenko, S.V. Stepkin, D.V. Shalunov // Радиофизика и радиоастрономия. — 2001. — Т. 6, № 1. — С. 21-31. — Бібліогр.: 38 назв. — англ. 1027-9636 http://dspace.nbuv.gov.ua/handle/123456789/122222 ru Радиофизика и радиоастрономия Радіоастрономічний інститут НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	Russian
description	The low-frequency carbon recombination lines (with wavelengths up to twenty five and even more meters) became important means of the low-density interstellar plasma diagnostics. An impressive amount of astrophysical information was obtained over the past twenty years the carbon lines were detected in the frequency range from 12 to 1400 MHz. Correspondingly, the maximum principal quantum numbers of the observed interstellar atoms are more than 800. The medium in the direction of Cassiopeia A is the best studied object, which forms such features. The volume of data for other galactic sources is also increasing. The observation of lines from such highly excited atoms is a unique and effective method for probing the physical conditions of the low-density interstellar plasma as well as the important means for the study of the physics of the high Ridberg state atoms.
format	Article
author	Konovalenko, A.A. Stepkin, S.V. Shalunov, D.V.
spellingShingle	Konovalenko, A.A. Stepkin, S.V. Shalunov, D.V. Low-Frequency Carbon Recombination Lines Радиофизика и радиоастрономия
author_facet	Konovalenko, A.A. Stepkin, S.V. Shalunov, D.V.
author_sort	Konovalenko, A.A.
title	Low-Frequency Carbon Recombination Lines
title_short	Low-Frequency Carbon Recombination Lines
title_full	Low-Frequency Carbon Recombination Lines
title_fullStr	Low-Frequency Carbon Recombination Lines
title_full_unstemmed	Low-Frequency Carbon Recombination Lines
title_sort	low-frequency carbon recombination lines
publisher	Радіоастрономічний інститут НАН України
publishDate	2001
url	http://dspace.nbuv.gov.ua/handle/123456789/122222
citation_txt	Low-Frequency Carbon Recombination Lines / A.A. Konovalenko, S.V. Stepkin, D.V. Shalunov // Радиофизика и радиоастрономия. — 2001. — Т. 6, № 1. — С. 21-31. — Бібліогр.: 38 назв. — англ.
series	Радиофизика и радиоастрономия
work_keys_str_mv	AT konovalenkoaa lowfrequencycarbonrecombinationlines AT stepkinsv lowfrequencycarbonrecombinationlines AT shalunovdv lowfrequencycarbonrecombinationlines
first_indexed	2025-07-08T21:21:05Z
last_indexed	2025-07-08T21:21:05Z
_version_	1837115288117575680
fulltext	Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1, ñòð. ??-?? © A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov, 2001 Low-Frequency Carbon Recombination Lines A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov Institute of Radio Astronomy of NAS of Ukraine 4 Chervonopraporna St., 61002 Kharkov, Ukraine E-mail: akonov@ira.kharkov.ua Received May 17, 2001 The low-frequency carbon recombination lines (with wavelengths up to twenty five and even more meters) became important means of the low-density interstellar plasma diagnostics. An impressive amount of astrophysical information was obtained over the past twenty years � the carbon lines were detected in the frequency range from 12 to 1400 MHz. Correspondingly, the maximum principal quantum numbers of the observed interstellar atoms are more than 800. The medium in the direction of Cassiopeia A is the best studied object, which forms such features. The volume of data for other galactic sources is also increasing. The observation of lines from such highly excited atoms is a unique and effective method for probing the physical conditions of the low-density interstellar plasma as well as the important means for the study of the physics of the high Ridberg state atoms. 1. Introduction Until the end of 1970�s the investigations of the astrophysical phenomenon of radio recombi- nation lines (RRL) were a privilege of the high- frequency radio astronomy. The observations of the hydrogen, helium, carbon, and some other RRLs carried out mainly at the frequencies above 1 GHz (corresponding to the principal quantum numbers 200)n > allowed to study various physical and kinematic parameters, such as the temperature, density, size, pressure, and the element abundance of a number of HII regions and their neighbor- hoods [1]. Naturally, the hydrogen lines turned to be the most informative, because they are formed by the most abundant element. It is ionized mainly by the strong UV radiation ( 912Å)λ < from O and B stars inside the Stromgren zones. The at- tempts to detect the hydrogen RRLs at the longer wavelengths (up to the metric range) were suc- cessful only for some selected objects. The low- est-frequency hydrogen line detected is the 352H α feature at 150 MHz, originating in the hot gas located in the direction of the Galactic Center [2]. In spite of that, the importance of the low-frequency RRL investigations became clear many years ago. According to the theoretical estimations [3], rather intensive (due to stimulat- ed emission) low-frequency ( 100 300ν = ÷ MHz) hydrogen lines are expected to be formed in cold diffuse ( ~ 100eT K, ~ 0.03eN cm�3) in- terstellar components, which are heated and ion- ized by cosmic and X-rays at the hydrogen ioniza- tion rate 15~10H −ξ s�1. Surprisingly, these lines have not been detected yet, indicating that the upper limit of the hydrogen ionization rate is 1710H −ξ < s�1. It is important to point out that as early as in 1960�s the founder of Ukrainian decametric ra- dio astronomy Professor S. Ya. Braude had pro- posed to search for the radio recombination lines (with 600n > ) at the decameter waves. Such a program was started on his initiative, after the radio telescope UTR-2 had been built. The detection of the spectral features (they turned out to be the RRLs of the strongly excited carbon atoms with ~ 630n ) in absorption at ex- tremely low frequencies ( 30ν < MHz) [4] with UTR-2 [5, 6] opened new ways of studying the low-density interstellar plasma by means of the A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov 2 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 methods of low and very low frequency radio spectroscopy. This brief review is devoted to a description of the most important stages and aspects of these investigations over the past twenty years. 2. General Properties of the Interstellar Medium and the Role of Carbon Atom 2.1. Low-Density Interstellar Plasma The interstellar medium (ISM) is an important component of galaxies. It is even difficult to enu- merate all the significant physical processes in it � a lot of them are really important for our under- standing of the Universe. The physical conditions and processes in the ISM are dramatically various, especially for the interstellar gas [7]. Therefore the investigations of the ISM are of a great interest for the astrophysics. The principal component of the ISM is the gas penetrated by the cosmic rays, magnetic fields, and electromagnetic radiation of all kinds. It is known that the structure of the interstellar gas is complex: the values of the temperature and den- sity (the physical parameters of the first interest) are very different [7]. It is important to note that practically all the gas is ionized, at least at a low rate. Outside the HII regions most part of the gas (completely or partially ionized) has the electron density 1eN < cm�3. The temperature can be ei- ther high ( 310eT > K) or low ( 100eT < K). This gas plays a significant role in the energetics, dy- namics, and evolution of the galactic matter. As an example, we can mention the first stage of a star birth (the Rayleigh � Taylor instability). It requires the presence of a magnetic field and only a small amount of electrons. One should empha- size that because of the very low emission mea- sure the experimental studies of such a low-den- sity plasma are very difficult in both the spectral lines and continuum. 2.2. Carbon Atoms and the Physical Processes in the ISM It is believed that most part of the electrons in the neighborhood of the hot interstellar gas is pro- duced by the carbon atoms (this suggestion was first made more than forty years ago). Carbon is the most abundant element 4(C H 3.7 10 )−= ⋅ among those having the ionization potential less than that of hydrogen ( C 11.2E = eV, H 13.6E = eV). The UV photons with 912Å < < 1100Åλ aris- ing in the O and B stars and propagating through the medium can ionize carbon almost complete- ly. Cooling of the gas is produced by the emission line corresponding to a fine structure transition of the carbon ions 2 2 3 2 1 2P P− with 0.0079E∆ = eV, 92T∆ = K, and 157 .λ = µ However, an analysis of the heating-cooling equilibrium gives the ki- netic temperature kT of only ~15 K. This is considerably less than the value obtained, for example, from the HI-line observations ( ~ 50 100kT ÷ K) [7]. Later on some new factors leading to heating and ionization in the cold components of ISM were proposed, in particular, cosmic rays (1 2÷ MeV) and the X-rays (~10 keV) [8]. This can yield the hydrogen ionization rate 15 H ~10−ξ s�1 and the electron density ~ 0.002 0.05eN ÷ cm�3. In these models carbon atoms also remain the main cooling element. Note that the expected intensities of the corresponding hydrogen RRLs ( 200 400)n = ÷ would be rather high, but they have not been de- tected yet. Perhaps, the absence of these lines can be attributed to the almost complete neutrality of hydrogen in the cold components of ISM. Of course, more careful investigations are necessary to clar- ify the problem. It is evident that the processes of heating the medium outside the HII region are complicated. In order to summarize the main determining fac- tors, we can mention the photoionization, the cos- mic rays, the photoelectric emission of small grains, the magnetic reconnection, the dissipation of the turbulence of the interstellar plasma, and the pho- toionization of the polycyclic aromatic hydrogen molecules [8-10]. A particular role of carbon as one of the main sources of the electrons and ions in the cold ISM should be stressed especially. The reasons deter- mining the significance of the carbon atom in the ISM are the following: � carbon is the most abundant element among those having the ionization potential less than that of hydrogen ( 4C H 3.7 10 ,−= ⋅ C 11.2E = eV, H 13.6E = eV); Low-Frequency Carbon Recombination Lines 3 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 � it is almost completely ionized in the diffuse interstellar gas; � it is the principal element, which determines the cooling and the thermostatic processes in the HI clouds due to the fine structure transition 2 2 3 2 1 2P P− with 157 ;λ = µ � most of the interstellar molecules contain the carbon atoms; � carbon plays a significant role in the gas- phase chemical reactions and effectively reflects various physical processes in the ISM. Neutral and ionized carbon can be observed in several ways. Some of them are presented in Fig. 1. The radio astronomical approach, parti- cularly the carbon RRL observations, may be the most promising one. 2.3. Carbon RRLs The first detection of the carbon RRLs was made at high frequencies [11]. After that they were observed in the directions of a number of HII regions [12, 13] simultaneously with the corre- sponding Hnα lines. As follows from the atomic physics, the mechanism of the carbon line forma- tion is similar to that of the hydrogen lines. The Rydberg�s formula gives: ( ) 2 2 2 1 1 1 ,e n a m cZ R M n n n     = − −  + ∆   ν where c is the velocity of light, Z is the effective nuclear charge, R is the Rydberg�s constant, em and aM are the electron and atom masses, n and n∆ are the principal quantum number and its in- crement. In the case considered only the isotopic shift is present (it is determined by the electron and atom mass ratio), so that the Cnα and Hnα line inten- sity is determined by the abundances of these elements inside the Stromgren zone. Correspond- ingly, the carbon lines should be weaker than the hydrogen ones by three orders of magnitude. However, the observed carbon line intensities are much stronger and are comparable to those of the hydrogen features. The carbon lines are believed to arise in the cold ( ~100eT K) gas lying at the periphery of HII region. It is the strong depen- dence of RRL optical depth on the temperature ( )2.5~1L eTτ that leads to unexpectedly high in- tensities of the observed carbon features. Some simple models of the objects, where the carbon RRLs can arise, are presented in Fig. 2. It should be stressed that for the objects of type 2 and 3 (those having no Hnα lines) at high frequencies the carbon RRLs were detected only in several selected regions [14]. For the type 3 objects (diffuse CII regions not connected with the HII ones) there is a single detection of Cnα at 1400 MHz [15]. These CII objects are associ- ated with the diffuse HI clouds, which are wide- spread over the Galaxy. Fortunately, another way exists, which opens absolutely new opportunities in the RRL studies. It is the radio spectroscopy at low frequencies. Fig. 1. Astrophysical methods of observation of the interstellar carbon Fig. 2. Possible ISM objects with ionized carbon (tri- angular arrows signify the ultraviolet quanta with 912Å 1100< λ < ): 1 � HII regions; 2 � dark dust clouds; 3 � diffuse clouds A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov 4 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 3. Low-Frequency RRLs in the Direction of Cassiopeia A 3.1 Actual Observational Data The carbon RRLs detected at decameter waves ( 30ν < MHz, 600n > ) in the direction of Cas A have the following peculiarities: � they are the first RRLs observed in ab- sorption; � their intensities are rather high � the inte- gration time necessary for their detection is about one hour; � line broadening is dramatically strong and surpasses all the examples observed in the astro- physics previously; � there is an evident variation of the line strength with the frequency; � association of these lines with rather cold gas in the diffuse CII regions, which are not connected with the HII ones, is a reasonable answer to the question where they arise, be- cause the Hnα lines are not detected in the corresponding directions. From the Shaver�s theory it follows that at higher frequencies such lines should turn into emission [6]. These features were first detected at 200ν > MHz [16]. The absorption-emission turnover reliably confirms the existence of the stimulated emission in the partially ionized low- density gas lying against a strong continuum radio source. Substantial variation of the line width and intensity, including the change of polarity (such a turnover is not observed at high frequen- cies), gives an excellent opportunity of diagnos- tics of the rarefied interstellar plasma. So it is not surprising that the Cnα lines are intensively stud- ied at many radio telescopes, including UTR-2 and RT-70 (Ukraine), GEE-TEE and Ooty (In- dia), DKR-1000 and RT-22 (Russia), Green Bank, the VLA, and Arecibo (USA), Effelsberg (Ger- many), Parkes (Australia) [17 and references therein]. Cas A is the strongest radio source, against which the RRLs are observed. In this case even for small antennas the antenna temperature suf- ficiently exceeds that of background. Thus, the same limb projection is investigated in all radio astronomical range, independently of the beam size (Fig. 3). Obviously, this source yields a unique opportunity of the most accurate deter- mination of the line and medium characteris- tics. So there is no exaggeration in saying that Cas A is the �corner-stone� of the low-fre- quency radio spectroscopy. The Cnα lines along the Cas A line of sight have been observed in the very wide frequency range, from 15 MHz ( ~ 800)n up to 1400 MHz ( ~160).n The total number of the features de- Fig. 3. Diagram of the low-frequency Cnα line observations. For S BT T :? L obs real ;             ∆ ∆ = = −τL L C C T T T T for BT :=ST cl B S a B Fobs real T T , T T     ⋅        Ω∆ ∆ = Ω + L L C C T T T T BT and FT are the source, background and frontal medium temperatures, respectively; s ,Ω clΩ and aΩ are the source, cloud and antenna solid angles; Lτ is the line optical depth; L C∆Τ Τ is the line relative intensity; the subscript �obs� means observed Low-Frequency Carbon Recombination Lines 5 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 tected comes to several tens. This is an unprec- edented achievement in comparison with the oth- er objects studied with RRLs. Quite recently the lowest frequency spectral lines ( ~ 812)n were detected near 12 MHz with the radio telescope UTR-2. The atoms with the highest principal quantum numbers of about 860 were observed using the β -lines near 20 MHz at the same instrument (see Fig. 4). 3.2. Problems of Interpretation The parameters of the low-frequency RRLs are mainly defined by the equations [3] given below in a qualitative form. The relative intensity is ( )2 5 2 1 , , , , , , , ,L e p e LR LR LD n n C T f N l T b T ∆ = ∆ν ∆ν ∆ν β the integral relative intensity ( )2 5 2 2d , , , , ,L L e p e n n C T I f N l T b Tν  ∆= ν = β    ∫ (1) the pressure broadening ( )2 0.5 5.2 3 , , ,LR e ef N T n∆ν ≈ (2) and the radiative broadening ( )5.8 4 , , .LR Rf T W n∆ν ≈ (3) Here eT and eN are the electron temperature and density, pl is the path length, n is the principal quantum number, RT and W are the radiation tem- perature and dilution factor, nb and nβ are the departure coefficient ( d(ln ) 1 , d e n n kT b h n β = − ν where k and h are the Boltsman and Plank constants). As is seen from the above equations, the line parameters are strongly dependent on the temperature and density. But the decisive fac- tors are the departure coefficients nb and ,nβ which determine a character of line behavior, including their polarity. The departure coeffi- cients appear to be crucial for model construc- tion. In 1980 it was shown that a mechanism of dielecronic-like recombination due to the fine structure transition 2 2 3 2 1 2P P− ( 92T∆ = K), can considerably modify the departure coeffi- cients of the carbon ion states [18]. Detailed calculations [19] showed that in the case of ~ 100eT K n nb β can reach the values of 10 100.÷ One of other possible mechanisms of n nb β mod- ification is the underpopulation of the high atomic levels ( 0nb → with n → ∞ ) [20, 21]. Let us remind that the classical hydrogenic-like recom- bination ( 1nb → with n → ∞ ) gives the value of n nb β about 1 for high n [3]. An exotic mecha- nism of the high-temperature ( 410eT > K) dielec- tronic recombination can provide 3~10n nb β for the heavy elements [22]. The range where d dn nl b n is negative is of a particular interest: at the lowest frequencies a strong amplification of the absorption lines takes place. In order to construct a comprehensive physical model of the medium, it is necessary to fit the calculation data obtained using formulas (1)-(3) with the experimental results. Moreover, other astro- physical information on the ISM properties, includ- ing, primarily, the data on the HI and molecular lines, the interstellar pressure, and the thermal equilibrium, must be taken into account [21]. Fig. 4. A series of carbon RRLs near 20 MHz A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov 6 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 At first, two types of the medium models were proposed: 1. The cold gas model with ~ 20eT K, ~ 0.3eN cm�3, and the hydrogenic-like level pop- ulation; CII regions are associated with the cold, clumping, and mainly molecular gas [23]. 2. The warm gas model with ~100eT K, ~ 0.05eN cm�3, and dielectronic-like recombina- tion; the regions of line formation are associated with the diffuse HI clouds and are diffuse them- selves [16, 19, 24]. One of the problems met is rather low accu- racy of the experimental data, especially at the lowest frequencies ( 500).n > The difficulty lies not only in the insufficient integration time but also in the very complicated determination of the broad Lorenzian wings of the features. Although some methods of data correction have been proposed [21], the datum accuracy improvement (described below) is of the first importance. In spite of the above mentioned points, on the basis of the al- ready obtained information it can be concluded that the warm gas model better fits to the exper- imental data. The anomalous in ters te l lar pressure ( 510> K⋅cm�3) and rather high value of the ob- served line width are also among the problems of interpretation, even in the case of optimum fitting of the line parameters to the physical conditions (the electron temperature and den- sity) [21]. So, there is a need for further de- velopment and improvement of the models. 3.3. New Physical Models In the paper [21] a new kind of models was proposed, which involve the mechanism of under- population of the high atomic levels. The increase of the derivative provides an agreement between the high line intensity at low frequencies and low intensity at the high ones even if the temperature and density are as low as 35 K and 0.05 cm�3, correspondingly. For this kind of models the H eN T factor of about 104 K⋅cm�3 becomes reasonable. However the calculated line width at low fre- quencies is lower than the observed one. In the paper [17] it was proposed to consider an additional broadening effect (3), caused by the non-thermal radio emission, simultaneously with the decrease of the distance between the line forming region and Cas A supernova remnant. This distance can be about 100 pc, if we take into account the dilution factor and accept the mini- mum possible, according to observed line widths, full non-thermal radiation temperature (~3200 K at 100 MHz), and the electron density. The best- fit warm gas model parameters are 75eT = K, 0.02eN = cm�3, and the emission measure 0.011EM = cm�6⋅pc [17]. It should be empha- sized that the size of the region along the line-of- sight is about 30 pc, that is sufficiently bigger than the linear size of Cas A (~7 pc). The very important result was obtained while comparing the high-resolution maps of the 274C α line distribution across the Cas A limb with the HI and CO data [17]. A better coinci- dence of the CII regions with the atomic hydro- gen distribution is evident. The inhomogeneity of the Cnα line forming region is also clear. Prob- ably, a small part of the ionized carbon (as com- pared to the whole CII region) is spread over the periphery of the molecular clumps [23] and the real medium parameters lie in the intervals 35 100eT = ÷ K and 0.01 0.1eN = ÷ cm�3. The recombination mechanism providing non-hydro- genic level population is always involved. It might be a dielectronic-like process as well. Owing to the observed amplification of absorption features, it is very important as yielding a unique opportu- nity of plasma diagnostics for a great number of the interstellar objects even when the electron density is less than 0.1 cm�3. 4. Observations of Low-Frequency Carbon RRLs in the Galaxy 4.1. Observational Data for a Number of Galactic Objects Successful observations of Cnα towards Cas A and a fruitful analysis of them motivated the search for such features in the directions to oth- er objects. The galactic background brightness temperature (~30000 K at 25 MHz) always exceeds the electron temperature of an investi- gated region throughout the Galaxy. However, the values of the observed relative line intensi- Low-Frequency Carbon Recombination Lines 7 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 ties (as it is seen in Fig. 3) are less than the real ones due to, in the first turn, the telescope beam dilution and the foreground emission (especially when the objects are distant). So the integration time needed for a reliable detection or the esti- mation of the upper limits on the line parameters, can reach several hundred hours. In spite of that, the promising perspectives of these investiga- tions are worse all the efforts. A search for the Cnα lines near 25 MHz ( ~ 640)n in various galactic objects has been carried out over the past two decades with the world largest decameter-wave array UTR-2 [25] and the 128-channel digital correlometer. The ef- fective area of UTR-2 is huge (~150 000 m2) and the angular resolution is high for a telescope of the decametric range ( ~ 30′ at 25 MHz) [25]. How- ever, owing to zero spacing problem, which is typi- cal for the T-shaped correlation telescope, and ex- pectedly large spatial sizes of the investigated ob- jects, the North � South arm of UTR-2 (the effec- tive area ~100000 m2, the beam size ~ 0.5 12°× ° ) was mainly used in this work. UTR-2 has a well developed and flexible structure that provides con- venient ways of sky mapping (for example, there can be five simultaneously operating beams and there is a possibility of array splitting into parts in any configuration) [26]. However, the small num- ber of the correlometer channels restricted the investigation potential. Now a considerable up- grade of the spectral equipment and methods lead- ing to improvement of the sensitivity, the interfer- ence immunity, and the reliability of measurements is under way at UTR-2 observatory. In the first turn, it would be very interesting to observe in the directions of the maxima of the HI column density ( 20 H 10N > cm�2) near the galactic plane (they could be determined, for example, from the HI maps and HI absorption line surveys [27, 28]), the HII regions, SNRs, dark dust clouds and giant molecular clouds. The positive results have been already obtained for many objects, in particular, NGC 2024; 75 ,l = ° 0 ;b = ° S140; DR-21; L1407; ρ Oph; M16; Per OB2; 35 ,l = ° 0 ;b = ° h17 ,α = 70 .δ = ° The relative intensities of the lines obtained are 4 35 10 1 10− −⋅ ÷ ⋅ and the line widths are 10 50÷ km/s. The radial velocities are in a good correspondence with the kinematic parameters of the Galaxy. The estimation of the region sizes across the line-of-sight is rather difficult due to relatively large beam of the North � South tele- scope arm. A more thorough interpretation is complicated by the absence of reliable data for the higher frequencies ( 30 300÷ MHz). It is evident that the low-frequency Cnα lines arise outside the regions forming the high-frequency ( 1ν > GHz) carbon lines, if any exists. 4.2. Inner Part of the Galaxy The investigations of the regions along the line- of-sight in the directions close to the Galactic Cen- ter have been carried out systematically with the radio telescopes in Parkes at 75ν = MHz [29], Garibidanur at 34.5ν = MHz, and Ooty at 328ν = MHz [30]. Thirty positions with the ga- lactic longitude lying in the interval of 145 342l = ° ÷ ° (with the step 5 15÷ °) were ob- served at 34.5 MHz. Thirty per cent of these directions gave the Cnα line detection. Among other positive results there are the measurements in the directions of 63 ,l = ° 75l = ° and DR-21 ( 82 ).l ≈ ° The data of the last two observations are in a good agreement with the UTR-2 data. The emission carbon lines at 328 MHz were de- tected in the galactic longitude range of 392 16.5 .l = ÷ ° The investigations of these re- gions at 75 MHz give positive result too. Not only the absorption Cnα lines but also the β and γ ones were detected. The most feasible estimate of the angular size of the regions given in the above mentioned works are in the range of 2 4 .÷ ° According to the VLA mapping at 330 MHz for 14l = ° and 0b = ° (it gave the upper limit of the line intensities rather than detec- tion) the clump sizes are more than 10′ even in the case when the distribution of CII in these regions is very inhomogeneous. It is important that the line width does not depend upon n for most of the directions in the frequency range 25 328÷ MHz ( 20 50L∆ν = ÷ MHz). At the same time, for the Cas A direction L∆ν changes from 5 to 70 km/s. Possibly, the line width is de- termined only by the systematical and turbulent gas movements. The supposed upper limit of the electron density is 0.3eN < cm�3. A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov 8 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 4.3. Interpretation of Results The interpretation of the experimental data is complicated by the fact that a number of physical parameters is unknown. The following combina- tions of the measured line characteristics and missing information are met when the data are interpreted (the Doppler and radiative broadening are supposed to be known and the medium is supposed to be homogeneous): 1. There are two equations ((1) and (2)) and three unknown values ( , , );e eN T S the beam width is less than the region size or the gas is projected onto a strong compact radio source (like Cas A if, of course, the cloud sizes are not smaller than its limb). The problem is not resolvable, when there are data only for one observed frequency, even the lowest one with the evident pressure broaden- ing. However, if the measurements are multi-fre- quency the solution can be found (it is better when max min 10 20ν ν = ÷ ) because the third equation appears: ( , , ).n n e eb f n T Nβ = 2. There is the only case for L1407 [31], when even one low-frequency observational point al- lows to determine all three physical parameters. If the carbon lines and the continuum free-free absorption arise in the same cold gas (there is no HII region) then the third equation for the contin- uum optical depth cτ appears to be: 1 .cC C T e T −τ∆ = − As the low-frequency continuum map exists, it is possible to determine the region size and the beam dilution rate. 3. The sizes of the CII regions for most part of the objects described in subsections 4.1 and 4.2 are unknown. The dependence of beam width upon frequency makes a problem as well. So, even approximate estimation of the region sizes (obtained, for example, by the measurements with the same antennas but using various aperture siz- es) is very important [32]. Nevertheless, in some cases it is possible to make significant estimations of max,eN min ,eT and minS even having one frequency of the decametric range [33]. In any case, it is important to use all the ac- cessible astrophysical data for construction of a reliable model (they are, for example, the HI and molecular lines, the continuum thermal and non- thermal radio emission, and the 157µ CII emis- sion line). Using the measurements at three frequencies for the inner part of the Galaxy as well as the angu- lar size estimation, the most feasible values of the medium properties were found. They are the fol- lowing: 40 100eT = ÷ K, 30.003 0.01 cm ,eN −= ÷ and the path length is more than several parsecs. For the direction of 0 ,l = ° 0b = ° the data from the papers [34-36] were also used. Thus, the low-fre- quency Cnα lines arise mainly in the diffuse rather warm CII regions associated with the HI gas where the conditions for dielectronic-like process exist. Probably, the lines at the lowest and higher frequencies ( 200ν > MHz) are formed in the regions with the conditions, which are different to some extent. This follows from a slight distinction in the velocities of the absorption and emission lines in several cases [35]. 5. Future Perspectives of Low-Frequency RRL Investigation 5.1. New Instrumentation and Methods During the last years the interest to the low- frequency radio astronomy has grown consider- ably. It is caused, in particular, by the new oppor- tunities opened by the low-frequency carbon RRL investigations. The radio telescope UTR-2 remains the most effective instrument at the frequencies less than 30 MHz. It is characterized by the biggest effec- tive area, the high directivity, the broad operating band, the electronic beam steering, and the multi- beam observation regimes. For the spectral in- vestigations there is no sensitivity limitation due to confusion effect, so that the sensitivity can be much better than for the other operating mode of the UTR-2. The recent upgrade of the preampli- fier system has provided the uninterrapted fre- quency range and a good interference immunity, Low-Frequency Carbon Recombination Lines 9 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 which is extremely important for the spectral line observations [37]. Many of the world radio telescopes have un- dergone substantial upgrade related to the oper- ation at the frequencies above 30 MHz. Further- more, new highly efficient radio telescopes have arisen. First of all one should mention the GMRT (Giant Meter Radio Telescope, India), which is very suitable for the RRL investigations at 50 1400÷ MHz. Without any doubt, the devel- opment of low-frequency carbon RRL radio spec- troscopy is among scientific motivations for build- ing the giant low-frequency arrays of a new ge- neration. The progress of modern digital electronics and computer techniques provides effective ways of developing the wide-band (up to several doz- ens of MHz) and multi-channel (up to many thou- sands) digital correlometers and spectral pro- cessors. Such a new correlometer with the maximum sampling rate of 60 MHz and 4096 channels was installed at the UTR-2 observato- ry. On the other hand, a 1-bit quantization used in the wide-band correlometers of this kind might lead to problems when strong hindering signals are present. Fortunately, the theoretical evalua- tion and special experiments have shown that with such devices the low-frequency radio spec- troscopy at 30ν < MHz is quite possible in good many cases. The frequency distance between the lines rap- idly decreases with the wavelength: 2 4 6 3 . RcZ n nn ∆ ν∆ν ≈ ≈ In the range of 20 30÷ MHz there are about 90 α-lines and several hundred of β-lines. Si- multaneous observation of such a huge number of features leads to a considerable increase of the measurement sensitivity and the correspond- ing dramatic decrease of the necessary integra- tion time [38]. The line broadening at the lowest frequencies is very high, and a careful analysis of the Voigt profiles is required. One of the effective ways of profile fitting is based on the well known equation: { }( ) ( ) { ( )} { ( )} ,L G F F L F Gν ∗ ν = ν ⋅ ν { ( )} ,LF L e−π∆ν τν = 2 2 2 4ln2{ ( )} , D F G e π ∆ν τ− ν = where �*� denotes the convolution procedure; F, F are the direct and back Fourier transforms; ( ),L ν ( )G ν are the Lorentzian and Gaussian profiles; ,L∆ν D∆ν are the Lorentzian and Gaus- sian line widths; τ is the argument of the corre- lation function (the time delay). Thus, this method lies in fitting the correla- tion functions measured by the digital corre- lometer directly. 5.2. Observational Programs Although for the Cas A direction the most comprehensive data are obtained, the further de- velopment of these studies is not excluded. There are promising ways of the accuracy measure- ment and the Voight profile fitting improvement, especially at the lowest frequencies. The mapping, as made with the VLA, but at the GMRT frequen- cies of 50 and 610 MHz, would be also important. Preliminary experiments with the UTR-2 gave the carbon RRL detection outside the Cas A limb (the shift was up to 4°), which corresponded to the more extended region of line formation. So further investigations of this kind, as well as the search for the other radio lines towards Cas A (e. g. NI at 26.13 and 15.67 MHz and NaI at 1770 MHz) at low and high frequencies are of a great interest and importance. The systematic search and study of the Cnα lines in the direction of the galactic plane at the very low ( 30ν < MHz) and low ( 330ν < MHz) frequencies are also very important. The results and estimations obtained show that the features discussed can be detected in a good many galac- tic regions with the already existing and coming radio telescopes, if one employs new more so- phisticated experimental means and methods. The values of the relative line intensity to be mea- sured are about 3 410 10 .− −÷ This sensitivity level provides a possibility to obtain a valuable infor- mation about the physical parameters and pro- cesses in the ISM. A. A. Konovalenko, S. V. Stepkin, D. V. Shalunov 10 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 6. Conclusion The carbon RRLs at low frequencies have opened new opportunities for the diagnostics of the rarified interstellar plasma and the study of the Rydberg atom physics. Although they are al- ready not exotic, their properties and the condi- tions in the regions of their formation remain mysterious to a great extent and are still very interesting for investigation. In spite of the numer- ous difficulties met during the line detection and interpretation, the low-frequency carbon RRLs yield unique information not available with the other astrophysical methods. Acknowledgments. Many thanks to Profes- sor S. Ya. Braude for his stimulation and support of these investigations, as well as to L. G. Sodin for active participation and discussions. This ac- tivity is supported in part by the grants INTAS 96 � 0183, 97 � 1964, and INTAS-CNES 97 � 1450. References 1. P.A. Shaver, ed. Radio Recombination Lines. Dordrecht, Reidel Publ. Co., 1980, 284 pp. 2. K. R. Anantharamaiah, H. E. Payne, D. Bhatacharya. In: Radio Recombination Lines: 25 Years of Investigations. Eds.: M.A. Gordon, R.L. Sorochenko. Dordrecht, Kluwer Acad. Publ., 1990, pp. 259-263. 3. P. A. Shaver. Pramana. 1975, 5, No. 1, pp. 1-28. 4. A. A. Konovalenko, L. G. Sodin. Nature. 1980, 283, No. 5745, pp. 360-361. 5. D. H. Blake, R. M. Crutcher, W. D. Watson. Nature. 1980, 287, No. 5782, pp. 707-708. 6. A. A. Konovalenko, L. G. Sodin. Nature. 1981, 294, No. 5837, pp. 135-136. 7. L. Spitzer. Physical Processes in the Interstellar Medium. New York, Wiley, 1978, 318 pp. 8. A. Dalgarno, R. A. McCray. Ann. Rev. A&A. 1972, 10, No. 2, pp. 375-426. 9. R. J. Reynolds. In: The Physics of the Interstellar and Intergalactic Medium, ASP Conference Series, 80. Eds.: A. Ferrara et al. San Francisco, ASP, 1995, pp. 388-391. 10. L. d�Hendecourt, A. Leger. A&A. 1987, 180, No. 1, pp. L9-L11. 11. P. Palmer, B. Zuckerman, H. Penfield, A. E. Lilley, P. G. Mezger. Nature. 1967, 215, No. 5096, pp. 40-41. 12. V. Pankonin, C. M. Walmsley, T. L. Wilson, P. Thomasson. A&A. 1977, 57, No. 3, pp. 341-349. 13. N. G. Kantharia, K. R. Anantharamaiah, W. M. Goss. ApJ. 1998, 504, No. 1, pp. 375-389. 14. R. L. Brown. In: Radio Recombination Lines. Ed. P.A. Shaver. Dordrecht, Reidel Publ. Co., 1980, pp. 127-140. 15. R. M. Crutcher. ApJ. 1977, 217, No. 2, pp. L109-L112. 16. H. E. Payne, K. R. Anantharamaiah, W. C. Erickson. ApJ. 1989, 341, No. 2, pp. 890-897. 17. N. G. Kantharia, K. R. Anantharamaiah, H. E. Payne. ApJ. 1998, 506, No. 2, pp. 758-772. 18. W. D. Watson, L. R. Western, R. B. Christensen. ApJ. 1980, 240, No. 3, pp. 956-961. 19. C. M. Walmsley, W. D. Watson. ApJ. 1982, 260, No. 1, pp. 317-324. 20. S. A. Gulyaev, S. A. Nefedov. Astron. Nachr. 1989, 310, No. 2, pp. 403-406. 21. H. E. Payne, K. R. Anantharamaiah, W. C. Erickson. ApJ. 1994, 430, No. 2, pp. 690-705. 22. P. A. Shaver. A&A. 1976, 46, No. 1, pp. 127-135. 23. R. L. Sorochenko, C. M. Walmsley. A&A Trans. 1991, 1, No. 1, pp. 31-46. 24. A. A. Konovalenko. Soviet Astron. Lett. 1984, 10, No. 11, pp. 846-852. 25. S. Ya. Braude, A. V. Megn, L. G. Sodin. Antennas. 1978, 26, No. 1, pp. 3-15. 26. A. A. Konovalenko. In: AGU Monograph 119. Washington, AGU, 2000, pp. 311-320. 27. W. B. Burton. A&A Suppl. Ser. 1970, 2, No. 4, pp. 261-289. 28. V. Radhakrishnan, W. M. Goss, J. D. Murray, J. W. Brooks. ApJ. Suppl. Ser. 1972, 24, No. 203, pp. 49-72. 29. W. C. Erickson, D. McConnel, K. R. Anantharamaiah. ApJ. 1995, 454, No. 1, pp. 125-133. 30. D. A. Roshi, K. R. Anantharamaiah. MNRAS. 1997, 292, No. 1, pp. 63-71. 31. A. A. Golynkin, A. A. Konovalenko. Sov. Astron. J. Lett. 1991, 17, No. 1, pp. 23-29. 32. A. A. Konovalenko. Sov. Astron. J. Lett. 1984, 10, No. 12, pp. 912-917. 33. A. A. Golynkin, A. A. Konovalenko. Sov. Astron. J. Lett. 1991, 17, No. 1, pp. 16-22. 34. A. Pedlar, R. D. Davies, L. Hart, P. A. Shaver. MNRAS. 1978, 182, No. 2, pp. 473-488. 35. K. R. Anantharamaiah, H. E. Payne, W. C. Erickson. MNRAS. 1988, 235, No. 1, pp. 151-160. 36. G. T. Smirnov, V. V. Kitaev, R. L. Sorochenko, A. F. Scheglov. In: Pushchino: Annual Session of Sci. Council of ASC, Collection of Reports, 1996, p. 3. 37. E. P. Abranin, Yu. M. Bruk, V. V. Zakharenko, A. A. Konovalenko. Radiophysics and Radio- astronomy. 1997, 2, No. 1, pp. 95-102. 38. A. A. Konovalenko. In: Radio Recombination Lines: 25 Years of Investigations. Eds.: M. A. Gordon, R. L. Sorochenko. Dordrecht, Kluwer Acad. Publ., 1990, pp. 175-188. Low-Frequency Carbon Recombination Lines 11 Ðàäèîôèçèêà è ðàäèîàñòðîíîìèÿ, 2001, ò. 6, ¹1 Íèçêî÷àñòîòíûå ðåêîìáèíàöèîííûå ëèíèè óãëåðîäà À. A. Êîíîâàëåíêî, Ñ. Â. Ñòåïêèí, Ä. Â. Øàëóíîâ Íèçêî÷àñòîòíûå ðåêîìáèíàöèîííûå ëèíèè óãëåðîäà (äëèíû âîëí äî 25 ìåòðîâ è áîëåå) ñòàëè óäîáíûì ñðåäñòâîì äèàãíîñòèêè ðàç- ðåæåííîé ìåæçâåçäíîé ïëàçìû. Â òå÷åíèå ïîñëåäíèõ 20 ëåò íàêîïëåí áîëüøîé îáúåì àñòðîôèçè÷åñêîé èíôîðìàöèè. Ëèíèè óãëåðî- äà óäàëîñü îáíàðóæèòü â äèàïàçîíå ÷àñòîò îò 12 äî 1400 ÌÃö, ÷òî ñîîòâåòñòâóåò ãëàâíûì êâàíòîâûì ÷èñëàì ìåæçâåçäíûõ àòîìîâ, ïðå- âûøàþùèì 800. Íàèáîëåå ïîëíàÿ èíôîðìà- öèÿ ïîëó÷åíà äëÿ ñðåäû â íàïðàâëåíèè ðàäèî- èñòî÷íèêà Êàññèîïåÿ À. Ïîñòîÿííî óâåëè÷è- âàåòñÿ îáúåì äàííûõ äëÿ äðóãèõ îáúåêòîâ Ãàëàêòèêè. Íàáëþäåíèå ñïåêòðàëüíûõ ëèíèé ñòîëü ñèëüíî âîçáóæäåííûõ àòîìîâ îêàçàëîñü óíèêàëüíûì è ýôôåêòèâíûì ìåòîäîì èññëå- äîâàíèÿ ôèçè÷åñêèõ ñâîéñòâ ðàçðåæåííîé ìåæçâåçäíîé ïëàçìû, à òàêæå âàæíûì ñðåä- ñòâîì èçó÷åíèÿ ôèçèêè ðèäáåðãîâñêèõ àòîìîâ. Íèçüêî÷àñòîòí³ ðåêîìá³íàö³éí³ ë³í³¿ âóãëåöþ Î. Î. Êîíîâàëåíêî, Ñ. Â. Ñòåïê³í, Ä. Â. Øàëóíîâ Íèçüêî÷àñòîòí³ ðåêîìá³íàö³éí³ ë³í³¿ âóãëå- öþ (äîâæèíè õâèëü äî 25 ìåòð³â òà á³ëüøå) ñòàëè çðó÷íèì çàñîáîì ä³àãíîñòèêè ðîçð³äæå- íî¿ ì³æçîðÿíî¿ ïëàçìè. Ïðîòÿãîì îñòàíí³õ 20 ðîê³â íàêîïè÷åíî âåëèêèé îáñÿã àñòðîô³çè÷- íî¿ ³íôîðìàö³¿. Ë³í³¿ âóãëåöþ âäàëîñÿ âèÿâèòè ó ä³àïàçîí³ ÷àñòîò â³ä 12 äî 1400 ÌÃö, ùî â³äïîâ³äàº ãîëîâíèì êâàíòîâèì ÷èñëàì ì³æçî- ðÿíèõ àòîì³â, ùî ïåðåâèùóþòü 800. Íàéïîâ- í³øó ³íôîðìàö³þ îòðèìàíî äëÿ ñåðåäîâèùà ó íàïðÿìêó ðàä³îäæåðåëà Êàññ³îïåÿ À. Ïîñò³é- íî çá³ëüøóºòüñÿ îáñÿã äàíèõ äëÿ ³íøèõ îá�ºêò³â Ãàëàêòèêè. Ñïîñòåðåæåííÿ ñïåêòðàëü- íèõ ë³í³é âèñîêîçáóäæåíèõ àòîì³â âèÿâèëîñÿ óí³êàëüíèì òà åôåêòèâíèì ìåòîäîì äîñë³ä- æåííÿ ô³çè÷íèõ âëàñòèâîñòåé ðîçð³äæåíî¿ ì³æçîðÿíî¿ ïëàçìè, à òàêîæ âàæëèâèì çàñî- áîì âèâ÷åííÿ ô³çèêè ð³äáåðã³âñüêèõ àòîì³â.

Low-Frequency Carbon Recombination Lines

Репозитарії

Схожі ресурси