Physical properties of substellar population in the young σ Orionis cluster
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Головна астрономічна обсерваторія НАН України
2005
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| Цитувати: | Physical properties of substellar population in the young σ Orionis cluster / V.J.S. Bejar // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 239-243. — Бібліогр.: 18 назв. — англ. |
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irk-123456789-796502015-04-04T03:01:47Z Physical properties of substellar population in the young σ Orionis cluster Bejar, V.J.S. MS3: Physics of Stars and Galaxies 2005 Article Physical properties of substellar population in the young σ Orionis cluster / V.J.S. Bejar // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 239-243. — Бібліогр.: 18 назв. — англ. 0233-7665 http://dspace.nbuv.gov.ua/handle/123456789/79650 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 |
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MS3: Physics of Stars and Galaxies MS3: Physics of Stars and Galaxies Bejar, V.J.S. Physical properties of substellar population in the young σ Orionis cluster Кинематика и физика небесных тел |
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Article |
| author |
Bejar, V.J.S. |
| author_facet |
Bejar, V.J.S. |
| author_sort |
Bejar, V.J.S. |
| title |
Physical properties of substellar population in the young σ Orionis cluster |
| title_short |
Physical properties of substellar population in the young σ Orionis cluster |
| title_full |
Physical properties of substellar population in the young σ Orionis cluster |
| title_fullStr |
Physical properties of substellar population in the young σ Orionis cluster |
| title_full_unstemmed |
Physical properties of substellar population in the young σ Orionis cluster |
| title_sort |
physical properties of substellar population in the young σ orionis cluster |
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Головна астрономічна обсерваторія НАН України |
| publishDate |
2005 |
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MS3: Physics of Stars and Galaxies |
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http://dspace.nbuv.gov.ua/handle/123456789/79650 |
| citation_txt |
Physical properties of substellar population in the young σ Orionis cluster / V.J.S. Bejar // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 239-243. — Бібліогр.: 18 назв. — англ. |
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Кинематика и физика небесных тел |
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2025-07-06T03:40:32Z |
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2025-07-06T03:40:32Z |
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1836867369957326848 |
| fulltext |
PHYSICAL PROPERTIES OF SUBSTELLAR POPULATION
IN THE YOUNG σ ORIONIS CLUSTER
V. J. S. Béjar
Istituto de Astrof́ısica de Canarias, GTC project
C Vı́a Láctea s/n, E-38200 La Laguna (Tenerife), Spain
e-mail: vbejar@iac.es
We review different searches for isolated brown dwarfs and planetary mass objects (M < 13MJup)
which we have conducted in the optical and infrared ranges, in the very young (age of 1–8 Myr),
nearby (d ∼ 350 pc) σ Orionis cluster. We describe the main characteristics of this substellar
population: spectral properties, effective temperature, spatial distribution, and the presence of
discs. We present an initial mass spectrum in the substellar domain.
INTRODUCTION
The separation between stellar and substellar objects is well defined. Substellar objects are those that are unable
to sustain the stable hydrogen burning in their interiors. According to evolutionary models, for solar metallicity,
this borderline is established in masses lower than ∼ 75MJup [1]. They are two main kinds of substellar objects:
brown dwarfs and planets, and their distinction is still a question to be discussed. Recently, several authors
have proposed to establish the frontier between brown dwarfs and planets at the minimum mass which can
sustain the deuterium fusion, so, for solar metallicity, planets are objects with masses below 12–13MJup [13]
and brown dwarfs are objects with masses in the range from 75 to 13MJup. In this paper we will follow this
criterion.
The σ Orionis cluster is located around a multiple star of the same name, in the south–west part of the Orion
belt. This multiple system, composed by an O9.5V, two B0.5V, and two A0 stars, belongs to the well known
Orion OB1b association. From the first discoveries of numerous population of very low-mass stars and brown
dwarfs [4, 14, 15], the σ Orionis cluster has become a paradigmatic place for the understanding of stellar and
substellar forming processes, and we can find stars in the total range of masses from 25M�, brown dwarfs and
objects down to a few Jupiter masses. It is one of the most interesting sites to investigate substellar objects
because of its youth, proximity, and low extinction. The Hipparcos satellite has measured a distance of 350 pc
to the central star [12]. The presence of Li in the atmosphere of pre-main sequence stars later than K7 restricts
the age of σ Orionis to be younger than 8 Myr, with a most probable age of 2–4 Myr [17]. The cluster also
seems to exhibit very little reddening [6, 11].
OPTICAL AND INFRARED SEARCHES FOR SUBSTELLAR MEMBERS IN THE CLUSTER
Our group has performed several studies in order to detect very low-mass stars and brown dwarfs in the cluster.
First searches were performed in the RIZ bands, where these cool objects emit most of their light in the optical
range. For these searches we used instruments like the CCD-camera on the IAC-80 telescope (Teide Observatory)
and the Wide Field Camera (WFC) mosaic on the Isaac Newton Telescope (INT, Roque de los Muchachos
Observatory). These surveys covered a total area of about 1 deg2 reaching an I-band completeness magnitude
of 22. From the colour–magnitude diagrams, we selected a sample of more than 200 very low-mass stars and
brown dwarf candidates in the I magnitude interval 14–22. According to theoretical evolutionary models,
they span the mass range from 0.3 down to 0.013M�, i.e., cover the whole brown dwarf domain. Figure 1
shows a typical I, R–I (top) and I, I–Z (bottom) colour–magnitude diagram, where the selected candidates are
indicated by solid circles. Completeness and limiting magnitudes are also shown. We have performed the follow-
up infrared photometry of these candidates in order to discriminate between bona fide cluster members and
reddened field objects. These studies confirm that the majority of our candidates (> 75%) follow the infrared
photometric sequence predicted by the models and they seem to be very-low mass stars and brown dwarfs of
the cluster.
c© V. J. S. Béjar, 2004
239
Figure 1. Optical colour–magnitude diagrams: I , R–I (top) and I , I–Z (bottom). Selected candidates are indicated by
solid circles. Completeness and limiting magnitudes indicated by dashed and solid lines, respectively, are also shown
In order to detect fainter and less massive objects, we performed deeper surveys in the optical (I-band)
and near-infrared (JH bands) regions. From the correlation of these data we have identified more than
15 candidates with J magnitudes fainter than 18, which, according to the models, are planetary mass objects,
i.e., M < 0.013M�. Figure 2 shows a J , I–J colour–magnitude diagram of the correlation of I-band data ob-
tained with the WFC and J-band data obtained with the ISAAC instrument on the Very Large Telescope
(VLT, Paranal Observatory). Bona fide candidates are indicated by solid circles. Open circles represent those
candidates that follow the expected spectral sequence of the cluster. The 3 and 10 Myr isochrones from the Lyon
group [1, 9] are also shown.
SPECTROSCOPIC CHARACTERIZATION OF SUBSTELLAR OBJECTS
We have obtained low-resolution (∼ 20 Å) optical spectra in the range 635–920nm for a sample of 30 brown
dwarfs candidates, using spectrographs like ISIS on the William Herschel Telescope (WHT) and LRIS on
240
SOri70
Figure 2. J vs. I–J colour–magnitude diagram. Selected candidates are indicated by solid circles. Open circles denotes
the candidates with the spectroscopic confirmation. The 3 and 10 Myr isochrones from the Lyon group [1, 9] are also
shown
the Keck I telescope. Their spectral types range from M6 to M8.5, showing the typical TiO and VO absorption
bands. Their effective temperature ranges from 3000 to 2500K. These objects also show spectral features
suggestive of youth, like strong emission of the Hα and weak alkaline lines [4]. Several of the candidates (∼ 5%)
show near infrared excesses that could be associated with the presence of discs [3, 6]. For a few of them, we
have obtained the higher resolution spectra, which also shows forbidden lines, thought to be associated with
outflow events [8, 17]. Our studies of photometric variability of brown dwarfs in σ Orionis [7] also show that
this process can be related with the presence of accretion discs in the similar way as Classical T Tauri stars.
We have also obtained low-resolution optical and infrared spectra for our planetary mass candidates, using
LRIS and NIRSPEC spectrographs on the Keck telescope and FORS on the VLT [2, 10, 16]. They have
spectral types from M9 to T6, and all except two follow the expected spectral sequence of the cluster. Their
effective temperature ranges from 2400 to 1000 K. Our faintest candidate, S Ori 70 (T6) has an estimated mass
of only 2–8MJup [18]. Figure 3 shows the optical spectra of a typical M and L object of the cluster. Late
M spectral types are characterized in the optical range by the presence of strong TiO and VO absorption bands.
In L spectral types, these bands disappear and alkaline lines and hidrures become stronger. In the infrared range,
the water bands become stronger from M to late L spectral types. T or “methane” objects are characterized
by the presence of methane bands in their spectra. The effective temperature of late M dwarfs span from 3000
to 2200K, L dwarfs are in the range 2200–1500K, and T dwarfs are between 1500 and 800K.
THE SPATIAL DISTRIBUTION AND SUBSTELLAR MASS FUNCTION
From the combination of our more extended surveys, we have obtained a census of the substellar population
of the σ Orionis cluster. The projected spatial distribution of these objects decay very fast from the centre of
the cluster, which is a clear indication of their association with the central multiple star. This distribution can
be represented by an exponential law with a characteristic radius of ∼ 1 pc.
Using evolutionary models, we have derived the masses of these objects and we have obtained the mass
spectrum of the cluster from the low-mass stars (0.2M�), covering the whole brown dwarf domain (0.075M� to
0.013M�). This is shown in Fig. 4. We find that the mass spectrum increases toward lower masses and can
be represented by a power-law, dN/dM ∼ M−α with an exponent α = 0.8. These results indicate that brown
dwarfs can be very common, although their contribution to the total mass of the cluster is not significant [5].
241
SOri 47 (L1.5)
SOri 29 (M6)
VOVOVO
TiOTiOTiOTiOTiO
CsICsINaIRbIRbIKIαH
Figure 3. Optical spectra of an M and L object in the σ Orionis cluster. Main spectral features are indicated
α=0.8
s
Figure 4. The mass spectrum of σ Orionis cluster. The best power-law fit (dN/dM ∼ M−0.8) in the brown dwarf
domain is also indicated in dashed line. Error bars correspond to Poissonian uncertainties
242
CONCLUSIONS
We have performed optical and infrared searches to detect a substellar population in the σ Orionis cluster.
We have found a numerous population of brown dwarfs and planetary mass objects down to 2–5 MJup.
The projected spatial distribution of brown dwarfs can be reproduced by an exponential law with a characteris-
tic radius of ∼ 1 pc. The mass spectrum is an increasing function in the substellar domain (dN/dM ∼ M−0.8),
but the contribution of substellar objects to the total mass of the cluster is less than 10%.
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