Ya. V. Pavlenko

Dynamical Masses of the Binary Brown Dwarf GJ 569 Bab

We have obtained new images and high-resolution (R ~ 22,400) near-infrared (1.2400-1.2575 μm) spectra of each component of the brown dwarf binary GJ 569 Bab using the adaptive optics facility of the Keck II telescope and the NIRSPEC spectrometer. These data have allowed us to improve the determination of the astrometric orbit and to measure radial velocities of the components. We have used the astrometric and spectroscopic measurements to derive the dynamical mass of each brown dwarf and the systemic velocity of the pair by means of a χ2 fitting technique. From various considerations, the mass of each component is likely in the range 0.034-0.070 M☉ (GJ 569 Bb) and 0.055-0.087 M☉ (GJ 569 Ba). This implies that the mass ratio q of the binary is greater than 0.4, the most likely value being q = 0.75-0.85. Adopting 0.072 M☉ as the most conservative location of the substellar limit for solar metallicity, our analysis confirms GJ 569 Bb as the first genuine brown dwarf known without any theoretical assumptions. We have compared the dynamical masses of GJ 569 Ba and Bb, and their effective temperatures and luminosities, to the predictions of state-of-the-art theoretical evolutionary isochrones, finding that models exhibit good performance in the regime of high substellar masses if the binary is about a few hundred million years old. However, the surface gravities of GJ 569 Ba (M8.5 V) and Bb (M9 V) derived from our spectral analysis (the observed data have been compared to the latest synthetic spectra) appear to be smaller than the values provided by the evolutionary models.

Improved HCN/HNC linelist, model atmospheres and synthetic spectra for WZ Cas

We build an accurate database of 5200 HCN and HNC rotation-vibration energy levels, determined from existing laboratory data. 20 000 energy levels in the Harris et al. (2002b) linelist are assigned approximate quantum numbers. These assignments, lab determined energy levels and Harris et al. (2002b) energy levels are incorporated in to a new energy level list. A new linelist is presented, in which frequencies are computed using the lab determined energy levels where available, and the ab initio energy levels otherwise. The new linelist is then used to compute new model atmospheres and synthetic spectra for the carbon star WZ Cas. This results in better fit to the spectrum of WZ Cas in which the absorption feature at 3.56 µm is reproduced to a higher degree of accuracy than has previously been possible. We improve the reproduction of HCN absorption features by reducing the abundance of Si to [Si/H] = –0.5 dex, however, +

Lithium and Hα in stars and brown dwarfs of σ Orionis

We present intermediate- and low-resolution optical spectra around Hα and Li I λ6708 A for a sample of 25 low mass stars and 2 brown dwarfs with confirmed membership in the pre-main sequence stellar σ Orionis cluster. Our observations are intended to investigate the age of the cluster. The spectral types derived for our target sample are found to be in the range K6–M8.5, which corresponds to a mass interval of roughly 1.2–0.02 M⊙ on the basis of state-of-the-art evolutionary models. Radial velocities (except for one object) are found to be consistent with membership in the Orion complex. All cluster members show considerable Hα emission and the Li I resonance doublet in absorption, which is typical of very young ages. We find that our pseudo-equivalent widths of Hα and Li I (measured relative to the observed local pseudo-continuum formed by molecular absorptions) appear rather dispersed (and intense in the case of Hα) for objects cooler than M3.5 spectral class, occurring at the approximate mass where low mass stars are expected to become fully convective. The least massive brown dwarf in our sample, S Ori 45 (M8.5, ~0.02 M⊙), displays variable Hα emission and a radial velocity that differs from the cluster mean velocity. Tentative detection of forbidden lines in emission indicates that this brown dwarf may be accreting mass from a surrounding disk. We also present recent computations of Li  I λ6708 A curves of growth for low gravities and for the temperature interval (about 4000–2600 K) of our sample. The comparison of our observations to these computations allows us to infer that no lithium depletion has yet taken place in σ Orionis, and that the observed pseudo-equivalent widths are consistent with a cluster initial lithium abundance close to the cosmic value. Hence, the upper limit to the σ Orionis cluster age can be set at 8 Myr, with a most likely value around 2–4 Myr.

Old Stars in Young Clusters: Lithium-depleted Low-Mass Stars of the Orion Nebula Cluster

We measured lithium in a sample of low-mass stars (~0.1-0.3 M☉) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding to nuclear ages between ~15 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R diagram, reinforcing our early findings. For the two other stars, the nuclear age is significantly larger than the isochronal one. Several Li-depleted stars display accretion activity, veiling, and emission lines. We discuss empirical evidence in favor of the old nuclear age and the implications on the star formation history of the Orion cluster.

Effective temperatures, rotational velocities, microturbulent velocities and abundances in the atmospheres of the Sun, HD 1835 and HD 10700★

We describe our procedure to determine effective temperatures, rotational velocities, microturbulent velocities, and chemical abundances in the atmospheres of Sun-like stars. We use independent determinations of iron abundances using the fits to the observed Fe I and Fe II atomic absorption lines. We choose the best solution from the fits to these spectral features for the model atmosphere that provides the best confidence in the determined log N(Fe), Vt , and v sin i .Computations were done in the framework of LTE. Blending effects were accounted for explicitly. First, we compute the abundance of iron for a set of adopted microturbulent velocities. In some cases, a few points of log N(Fe I) = log N(Fe II) can be found. To determine the most selfconsistent effective temperature and microturbulent velocity in any star’s atmosphere, we used an additional constraint where we minimise the dependence of the derived abundances of Fe I and Fe II on the excitation potential of the corresponding lines. Using this procedure we analyse the spectra of the Sun and two well known solar type stars, HD1835 and HD10700 to determine their abundances, microturbulent velocity and rotational velocity. Our approach allows us to determine self-consistent values for the effective temperatures, abundances, Vt and v sin i . For the Sun we obtain the best agreement for a model atmosphere of Teff /log g/[Fe/H] = 5777/4.44/0.0, iron abundances and microturbulent velocities of log N(Fe) =4.44, Vt = 0.75 km/s, for the Fe I lines, and log N(Fe) = -4.47, Vt = 1.5 km/s for the Fe II lines. Furthermore, abundances of other elements obtained from the fits of their absorption features agree well enough (± 0.1 dex) with the known values for the Sun. We determined a rotational velocity of v sin i = 1.6 ± 0.3 km/s for the spectrum of the Sun as a star. For HD1835 the self-consistent solution for Fe I and Fe II lines log N(Fe)=+0.2 was obtained with a model atmosphere of 5807/4.47/+0.2 and microturbulent velocity Vt = 0.75 km/s, and leads to v sin i = 7.2 ± 0.5 km/s. For HD10700 the self-consistent solution log N(Fe) = -4.93 was obtained using a model atmosphere of 5383/4.59/-0.6 and microturbulent velocity Vt = 0.5 km/s. The Fe I and Fe II lines give rise to a v sin i = 2.4 ± 0.4 km/s. Using the Teff found from the ionisation equilibrium parameters for all three stars, we found abundances of a number of other elements: Ti, Ni, Ca, Si, Cr. We show that uncertainties in the adopted values of Teff of 100 K and Vt of 0.5 km/s change the abundances of elements up to 0.1 and 0.2 dex respectively. Galactic abundances variations can generally be larger than this measurement precision and therefore we can study abundance variations throughout the Galaxy.

A H13CN/HN13C linelist, model atmospheres and synthetic spectra for carbon stars

‘The definitive version is available at www.blackwell-synergy.com’. Copyright Blackwell Publishing DOI: 10.1111/j.1365-2966.2008.13642.x

The identification of HCN and HNC in carbon stars: model atmospheres, synthetic spectra and fits to observations in the 2.7–4.0 μm region

Model carbon star atmospheres and synthetic spectra have been calculated using the recent HCN/HNC vibration rotation linelist of Harris et al. 2002b, ApJ, 578, 657. The calculations are repeated using only HCN lines and show that HNC has a significant effect upon the temperature, density and optical depth of a stellar atmosphere. We fit synthetic spectra in the 2.7-4.0 µm region to observed ISO spectra of the carbon stars WZ Cas and TX Psc obtained by Aoki et al. 1998, A&A, 340, 222. These fits allow us to identify absorption by HNC in the spectrum of WZ Cas at 2.8-2.9 µm, and to determine new independent estimates of effective temperature and log(NC/NO) . The findings reported here indicate that absorption by both HCN and HNC is needed to fully explain the observed stellar spectra and represent the first

Lithium lines in the spectra of M dwarfs: UX Tau C

The results of numerical modeling of lithium lines in the spectra of M dwarfs are discussed. The behavior of the lithium lines relative to the local pseudocontinuum formed by molecular bands is analyzed as a function of model atmosphere parameters: effective temperature Teff, gravity logg, and metallicity μ The molecular opacity was computed using the just overlapping line approximation (JOLA) and “line-by-line” methods. The pseudo-equivalent widths of lithium lines depend appreciably on metallicity μ and weakly on T{nteff}. The lithium abundance in the atmosphere of UX Tau C is redetermined. Previous studies underestimated the lithium abundance in this star as a result of the use of insufficiently accurate molecular-line lists. The new lithium abundance log N (Li)=3.2±0.3was derived by comparing the observed profiles of the 670.8 nm resonance doublet lines with profiles calculated using the new TiO line list of Plez.1 The new abundance agrees with the atmospheric lithium abundances of the other two components in the stellar system, providing further evidence that the three stars in the UX Tau system have the same age. A comparison of the observed spectra of UX Tau C near the lithium resonance doublet (665–680 nm) with spectra computed using JOLA and line-by-line methods suggests that the list of Plez is the best currently available.

First detection of a lithium rich carbon star in the Draco dwarf galaxy: Evidence for a young stellar population

We present a spectroscopic study of D461, a giant star belonging to Draco dwarf spheroidal galaxy. From spectral synthesis in LTE we derive a lithium abundance of log � (Li) = 3.5±0. 4a nd aC /O ratio between 3 and 5. This is the first detection of a lithium rich C-star in a dwarf spheroidal galaxy. Basing on stellar models of appropriate chemical composition, we show that a similar C enrichment is compatible with that expected for a low mass low metallicity thermally pulsing AGB star, undergoing few third dredge up episodes. The position in the log g-log Teff diagram of D461 is also compatible with this theoretical scenario. In particular, the low effective temperature, lower than that expected for a low metallicity giant star, is a consequence of the huge increase of the envelope opacity occurring after the carbon dredge up. The Li enrichment may be explained if a deep circulation would take place during the interpulse period, the so called cool bottom process. In spite of the low resolution of our spectra, we derive a lower limit for the carbon isotopic ratio, namely 12 C/ 13 C > 40, and a constraint for the Ba abundance, namely 0.5 < (Ba/Fe) < 2. The proposed scenario also fits these further constraints. Then, we estimate that the mass of D461 ranges between 1.2 and 2 M� , which corresponds to an age ranging between 1 and 3 Gyr. We conclude that this star is more massive and younger than the typical stellar population of Draco.

S Ori J053825.4 024241: A Classical T Tauri-like object at the substellar boundary

We present a spectrophotometric analysis of SOri J053825.4 024241, a candidate member close to the substellar boundary of the young (1-8Myr), nearby (�360pc) � Orionis star cluster. Our optical and near-infrared photometry and low-resolution spectroscopy indicate that SOri J053825.4 024241 is a likely cluster member with a mass estimated from evolutionary models at 0.06 +0.07 −0.02 M⊙, which makes the object a probable brown dwarf. The radial velocity of SOri J053825.4 024241 is similar to the cluster systemic velocity. This target, which we have classified as an M6.0±1.0 low-gravity object, shows excess emission in the near-infrared and anomalously strong photometric variability for its type (from the blue to the J band), suggesting the presence of a surrounding disc. The optical spectroscopic observations show a continuum excess at short wavelengths and a persistent and resolved Hemission (pseudo-equivalent width of � 250u in addition to the presence of other forbidden and permitted emission lines, which we interpret as indicating accretion from the disc and possibly mass loss. We conclude that despite the low mass of SOri J053825.4 024241, this object exhibits some of the properties typical of active classical T Tauri stars.