Ultracool dwarfs
We present results of modelling of spectra of M-, L-, T-dwarfs. Theoretical spectra are fitted to observed spectra to study the main parameters of the low-mass objects beyond the bottom of the Main Sequence. Application of the “lithium” and “deuterium” tests for assessment of ultra-cool dwarfs is di...
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Головна астрономічна обсерваторія НАН України
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irk-123456789-796512015-04-04T03:02:08Z Ultracool dwarfs Pavlenko, Ya.V. MS3: Physics of Stars and Galaxies We present results of modelling of spectra of M-, L-, T-dwarfs. Theoretical spectra are fitted to observed spectra to study the main parameters of the low-mass objects beyond the bottom of the Main Sequence. Application of the “lithium” and “deuterium” tests for assessment of ultra-cool dwarfs is discussed. 2005 Article Ultracool dwarfs / Ya.V. Pavlenko // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 244-249. — Бібліогр.: 44 назв. — англ. 0233-7665 http://dspace.nbuv.gov.ua/handle/123456789/79651 en Кинематика и физика небесных тел Головна астрономічна обсерваторія НАН України |
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MS3: Physics of Stars and Galaxies MS3: Physics of Stars and Galaxies Pavlenko, Ya.V. Ultracool dwarfs Кинематика и физика небесных тел |
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We present results of modelling of spectra of M-, L-, T-dwarfs. Theoretical spectra are fitted to observed spectra to study the main parameters of the low-mass objects beyond the bottom of the Main Sequence. Application of the “lithium” and “deuterium” tests for assessment of ultra-cool dwarfs is discussed. |
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Article |
| author |
Pavlenko, Ya.V. |
| author_facet |
Pavlenko, Ya.V. |
| author_sort |
Pavlenko, Ya.V. |
| title |
Ultracool dwarfs |
| title_short |
Ultracool dwarfs |
| title_full |
Ultracool dwarfs |
| title_fullStr |
Ultracool dwarfs |
| title_full_unstemmed |
Ultracool dwarfs |
| title_sort |
ultracool dwarfs |
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Головна астрономічна обсерваторія НАН України |
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2005 |
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MS3: Physics of Stars and Galaxies |
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http://dspace.nbuv.gov.ua/handle/123456789/79651 |
| citation_txt |
Ultracool dwarfs / Ya.V. Pavlenko // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 244-249. — Бібліогр.: 44 назв. — англ. |
| series |
Кинематика и физика небесных тел |
| work_keys_str_mv |
AT pavlenkoyav ultracooldwarfs |
| first_indexed |
2025-07-06T03:40:35Z |
| last_indexed |
2025-07-06T03:40:35Z |
| _version_ |
1836867373131366400 |
| fulltext |
ULTRACOOL DWARFS
Ya. V. Pavlenko
Main Astronomical Observatory, NAS of Ukraine
27 Akademika Zabolotnoho Str., 03680 Kyiv, Ukraine
e-mail: yp@mao.kiev.ua
We present results of modelling of spectra of M-, L-, T-dwarfs. Theoretical spectra are fitted
to observed spectra to study the main parameters of the low-mass objects beyond the bottom of
the Main Sequence. Application of the “lithium” and “deuterium” tests for assessment of ultra-cool
dwarfs is discussed.
INTRODUCTION
Population of ultracool (UC) dwarfs occupies the right-right-bottom quadrant below the bottom of the conven-
tional Main Sequence. A lot of UC dwarfs was discovered after 1995 (see [3] and [5] for reviews). Basically, we
can define at least three different populations of ultracool dwarfs:
— Low mass stars (LMS). Hydrogen burns in their core.
— Brown dwarfs (BD). Hydrogen cannot burn in their core. Their existence was predicted by Kumar
[17, 18]. Later investigations show that lithium burns inside the brown dwarfs of 55Mj < M < 75Mj (see [9] for
more details). Here Mj is mass of Jupiter: 1Mj = 0.001M�. The first brown dwarfs Teide1 and Gl 229B were
discovered by groups of Rebolo et al. [38] and Nakajima et al. [24], respectively. Deuterium should be depleted
in atmospheres of brown dwarfs.
— Planets (M < 13MJ) preserve deuterium (and lithium) during their evolution [40].
First spectral classifications of UC dwarfs were provided by Kirkpatrick et al. [16] and Mart́ın et al. [21].
Today, we can assess their spectra (see libraries of spectra in [10] or [13]):
— M-dwarfs (GJ406, VB10, VB8, etc.). TiO dominates in their spectra.
— L-dwarfs (GD169B, Kelu-1, 2MASS 0920+35, etc.). K and Na lines are the main features here [27, 28],
Ti and V atoms are partially bound into dust particles.
— T-dwarfs (Gl 229B, SDSS 0151, SDSS 1110, etc.). Their infrared spectra contain CH4 lines.
— planets (see a list of discovered planets at web [12], and references therein). The first confirmed discovery
of the planetary system 51 Peg was carried out by Mayor & Queloz [23] (see Marcy et al. [20]).
M-, L-, T-dwarfs are of different effective temperatures and masses. Still, “The Main Sequence” for brown
dwarfs and L-, T-dwarfs forms the approximately horizontal line (Jupiter is on the left side of the radii-masses
plot, see [6]) – the dependence of radii of UC dwarfs on mass is extremely weak due to the degeneracy of the gas
in their cores. As result, sizes of old brown dwarfs, L-dwarfs and Jupiter are comparable.
As was noted by Zapatero Osorio (private communication) depending on age, T-dwarfs can be brown dwarfs
(if they are old) or “planetary objects” (their masses are below the deuterium burning limit, if they are young).
Hence, very young T-dwarfs do not burn deuterium. Then, giant planets around stars have been found by
indirect techniques. Young objects, which are a few times more massive than Jupiter, have been identified using
direct imaging techniques. They are characterized by ultracool atmospheres (L and T types). These objects
are free-floating in star-forming regions and very young clusters. This poses a challenge to the current theories
of stellar and planetary formation (see Proc. of IAUS 211 [5]).
Different UC dwarfs are of different structure as well:
— inside the LMS we have the core with a hydrogen burning zone,
— brown dwarfs burn deuterium, the most massive BDs (55MJ < M < 75MJ) burn lithium within short
time scales (see refs in [19]).
— planets are only objects without any nuclear burning processes. They preserve deuterium and lithium
from times of their formation.
c© Ya. V. Pavlenko, 2004
244
Figure 1. Profiles of resonance lines of K I (λλ 766.6, 770.1 nm) and Na I (λλ 589.1, 589.7 nm) calculated in the frame
of the collisional broadening theory (the van der Waals’ broadening) for the 1200/5.0 C-model atmosphere of Tsuji [41]
(see [30] for more details)
MODELS OF FORMATION OF SPECTRA OF ULTRACOOL DWARFS
To model spectra and spectral energy distributions (SEDs) of ultracool dwarfs we should take into account
a few complicate processes which govern the physical state of their atmospheres:
• Dust formation processes. Due to low temperatures and a high pressure regime some molecular (and
atomic) species are bound in different grain particles (see [42]). Indeed, molecular bands of VO and TiO
are weaker in the L-dwarfs spectra in comparison with the M-dwarfs ones.
• Damping of K and Na lines. Resonance doublets of K and Na form the most impressive features in spectra
of L-dwarfs. Formally, a calculated equivalent widths of these lines can be of about few kÅ (see Fig. 1
and [28, 34] for more details).
• Dust opacities. Importance of taking into account of dust opacities by numerical modelling the spectra
of L- and T- dwarfs was shown by Pavlenko et al. [34]. Basically, the problem of the dust opacities
in the L-dwarf atmosphere is rather complicate – we should take into account absorption/scattering by
particles of various composition, sizes, orientations. Moreover, recent studies provide some evidence for
a cloudy structure of dust layers in atmospheres of L-dwarfs (see materials of IAUS 211 [5]).
OPTICAL SPECTRA: K AND NA LINES
Resonance lines of Na I (λλ 589.1, 589.7 nm) and K I (λλ 766.6, 770.1 nm) are very strong in spectra of UC
dwarfs [26], because the majority of alkali atoms exists there as neutral atoms. The Na I resonance lines are
stronger because log N(Na) > log N(K) in atmospheres of most stars.
Lines of alkali metals observed in UC dwarfs spectra are pressure broadened. The extremely strong broad-
ening of the K and Na resonance lines provides an serious problem for their modelling. We can use for their
wings modelling the traditional approach based on collisional interactions between atoms of K and Na and H,
He and molecule only for qualitative analysis [29].
The more sophisticated approaches based on the quantum-chemical consideration of the impact of potential
fields provided by different species on levels of K and Na were recently proposed by various groups (see [1]
and [7]).
On the other hand, in atmospheres of L-dwarfs the dust absorbs/scatters photons in a wide spectral region.
The dust opacity affects the total spectral distribution (see [34] for more details). Perhaps, for the core and
near wings of resonance lines K I and Na I we can still use the collisional approach (see ibid).
INFRARED SPECTRA: H2O BANDS
Water bands cover the wide regions of infrared spectra of UC dwarfs (see [32] and the paper by Lyubchik et al.
on this session). For a long time, the computation of the most complete lists of H2O is the real challenge
245
for theoretical physics (see a review in [31]). In general, the incompleteness of the water line lists used for
the numerical analysis of infrared spectra of UC dwarfs can complicate the calculation of stellar spectra because:
– the outer layers of model atmospheres calculated with an incomplete line lists of H2O are “too hot”.
– results of spectral synthesis can be affected by the incompleteness of the H2O lists.
Water bands in the IR are of interest for different topics. The infrared CO band at 2.3 and 4.5 micron can
be used for determination of basic parameters of UC dwarfs: abundances, effective temperatures, rotational
velocities (see [14, 32]). For their theoretical modelling the use of a reliable list of H2O lines is of crucial
importance (see [15] for more details).
LITHIUM TEST
“Lithium test” was proposed by Rebolo et al. [37] to identify brown dwarfs from the population of LMS.
Before 1995 L- and T-dwarfs were not known, and the main attention was paid for the low-gravity M-dwarfs.
They suggested that at least a part of low-mass dwarfs in young open clusters should preserve their lithium.
Observation of lithium lines in spectra of late M-dwarfs provides the direct evidence for their substellar nature.
Pavlenko et al. [33] showed that lithium lines can be detected in spectra of brown dwarfs despite of severe
blending of the atomic lines by molecular bands. Later lithium lines were really found in spectra of some brown
dwarfs (Teide1 [36], Kelu-1 [39], etc. see [3]).
On the other hand, observation of lithium lines in spectra of late-type low gravity dwarfs of open clusters
provide the information about their age. Due to theoretical predictions (see refs. in [43]) the smallest objects
should be cooled very quickly, i.e., within time scales of a few Myr. Still young, i.e., low gravity dwarfs
of ages 3–5 Myr preserve their lithium as well. In Fig. 2 results of determination of lithium abundances in
atmospheres of the low-mass dwarfs of the open cluster σ Ori are showed. Note, these results are based on
analysis of pseudoequivalent widths of lithium lines (see [27] and [43] for more details) – measurements of
the pseudoequivalent widths are provided in respect to the local pseudocontinuum formed by molecular lines.
Perhaps, the determination of masses of brown dwarfs is the main problem. Fortunately, brown dwarfs often
form binary systems. The study of the low-mass objects is of special interest. First observations of GJ569B
provide some evidences for its substellar nature (see refs in [19]. However, later observations of Mart́ın et al. [22]
on the Keck Telescope show that GJ569B is a double system – GJ569Ba and GJ569Bb are orbiting with period
Figure 2. Comparison of pseudoequivalent widths (pEW) of the lithium resonance doublet lines 670.8 nm calculated
for log N(Li) = 3.2 with the observed ones in spectra of young dwarfs of the σ Ori cluster. The TiO line list of Plez [35]
and NextGen model atmospheres [11] of solar metallicity [2] were used in theoretical computations. Solid and dashed
lines in the left part of the plot indicate the conventional curves of the growth of the lines computed for log N(Li) = 3.2
and 2.0, respectively. Open circles and open triangles indicate sources with Hα emission of pEW > 1 nm and objects
with forbidden emission lines, respectively (see Zapatero Osorio et al. [43] for more details)
246
0
2e-8
4e-8
6e-8
8e-8
1e-7
1.2e-7
1.4e-7
1.6e-7
1.8e-7
2e-7
1 2 3 4 5 6 7 8 9 10
F
lu
x
Wavelength in microns
H2O, D/H = 0
HDO, D/H = 1e-3
Figure 3. Calculated spectra of H2O and HDO for various ratios D/H. Calculations were made for the model atmosphere
1200/5.0 by Tsuji (1998) and the AMES line lists [25], with step 0.5 Å (see [31] for more details)
892 ± 25 days [19]. The lithium test for this system is of crutial importance. However, in this case we should
manage a combine spectrum formed in atmospheres of both components of different masses.
Later the application of the “lithium test” was discussed for L-dwarfs and even T-dwarfs (see [34]). Indeed,
lithium lines were observed in spectra of some UC dwarfs.
DEUTERIUM TEST
In the cores of ultracool dwarfs, the effects of correlations between ions dominate and cause the lowering
the Coloumb barrier between particles (see [9] for more details). Still, temperatures in the interiors of UC dwarfs
of mass M < 13MJ cannot be high enough (T < 0.5 MK) to initiate there a nuclear burning of deuterium.
Béjar et al. [4] propose to use observations of deuterium lines contained features to determine the ages/masses
ratio of the smallest UC dwarfs. The task is very difficult in both theoretical and observational aspects.
The simplest case which would be proposed consists of the analysis of HDO/H2O lines in the IR spectra of
UC dwarfs. However, HDO lines are very blended by H2O lines [31]. On the one hand, we should have the very
accurate lists of both H2O and HDO lines. Observed intensities of HDO lines cannot exceed a few per cent (see
ibid and [9]). Moreover, the IR spectrum of UC dwarfs should contain lines of other polyatomic species (CH4
and others). These factors make greater demands of the capacity of observational facilities and the quality of
theoretical data to identify and to carry the analysis of HDO lines in spectra of UC dwarfs.
Acknowledgements. I thank Maria Rosa Zapatero Osorio (LAEFF, Spain) for her highlighted and helpful
comments. I am grateful to my collaborators and co-authors Hugh R. A. Jones (Univ. of Hertfordshire, UK),
Rafael Rebolo, Mart́ın Eduardo, Vı́ctor J. S. Béjar (IAC, Spain) for their contribution.
My investigations were supported by a Small Research Grant of AAS and a Royal Society travel grant as
well as travel grants from Liverpool University (UK).
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