Hydrogen Burning Minimum Mass Research Articles

Parallaxes of Southern Extremely Cool objects III: 118 L and T dwarfs

We present new results from the Parallaxes of Southern Extremely Cool dwarfs program to measure parallaxes, proper motions and multi-epoch photometry of L and early T dwarfs. The observations were made on 108 nights over the course of 8 years using the Wide Field Imager on the ESO 2.2m telescope. We present 118 new parallaxes of L \& T dwarfs of which 52 have no published values and 24 of the 66 published values are preliminary estimates from this program. The parallax precision varies from 1.0 to 15.5 mas with a median of 3.8 mas. We find evidence for 2 objects with long term photometric variation and 24 new moving group candidates. We cross-match our sample to published photometric catalogues and find standard magnitudes in up to 16 pass-bands from which we build spectral energy distributions and H-R diagrams. This allows us to confirm the theoretically anticipated minimum in radius between stars and brown dwarfs across the hydrogen burning minimum mass. We find the minimum occurs between L2 and L6 and verify the predicted steep dependence of radius in the hydrogen burning regime and the gentle rise into the degenerate brown dwarf regime. We find a relatively young age of 2 Gyr from the kinematics of our sample.

Primeval very low-mass stars and brown dwarfs – III. The halo transitional brown dwarfs

We report the discovery of an esdL3 subdwarf, ULAS J020858.62+020657.0, and a usdL4.5 subdwarf, ULAS J230711.01+014447.1. They were identified as L subdwarfs by optical spectra obtained with the Gran Telescopio Canarias, and followed up by optical-to-near-infrared spectroscopy with the Very Large Telescope. We also obtained an optical-to-near-infrared spectrum of a previously known L subdwarf, ULAS J135058.85+081506.8, and reclassified it as a usdL3 subdwarf. These three objects all have typical halo kinematics. They have $T_{\rm eff}$ around 2050$-$2250 K, $-$1.8 $\leq$ [Fe/H] $\leq -$1.5, and mass around 0.0822$-$0.0833 M$_{\odot}$, according to model spectral fitting and evolutionary models. These sources are likely halo transitional brown dwarfs with unsteady hydrogen fusion, as their masses are just below the hydrogen-burning minimum mass, which is $\sim$ 0.0845 M$_{\odot}$ at [Fe/H] = $-$1.6 and $\sim$ 0.0855 M$_{\odot}$ at [Fe/H] = $-$1.8. Including these, there are now nine objects in the `halo brown dwarf transition zone', which is a `substellar subdwarf gap' that spans a wide temperature range within a narrow mass range of the substellar population.

Primeval very low-mass stars and brown dwarfs – II. The most metal-poor substellar object

SDSS J010448.46+153501.8 has previously been classified as an sdM9.5 subdwarf. However, its very blue $J-K$ colour ($-0.15 \pm 0.17$) suggests a much lower metallicity compared to normal sdM9.5 subdwarfs. Here, we re-classify this object as a usdL1.5 subdwarf based on a new optical and near-infrared spectrum obtained with X-shooter on the Very Large Telescope. Spectral fitting with BT-Settl models leads to $T_{\rm eff}$ = 2450 $\pm$ 150 K, [Fe/H] = $-$2.4 $\pm$ 0.2 and log $g$ = 5.5 $\pm$ 0.25. We estimate a mass for SDSS J010448.46+153501.8 of 0.086 $\pm$ 0.0015 M$_{\odot}$ which is just below the hydrogen-burning minimum mass at [Fe/H] = $-$2.4 ($\sim$0.088 M$_{\odot}$) according to evolutionary models. Our analysis thus shows SDSS J0104+15 to be the most metal-poor and highest mass substellar object known to-date. We found that SDSS J010448.46+153501.8 is joined by another five known L subdwarfs (2MASS J05325346+8246465, 2MASS J06164006$-$6407194, SDSS J125637.16$-$022452.2, ULAS J151913.03$-$000030.0 and 2MASS J16262034+3925190) in a 'halo brown dwarf transition zone' in the $T_{\rm eff}-$[Fe/H] plane, which represents a narrow mass range in which unsteady nuclear fusion occurs. This halo brown dwarf transition zone forms a 'substellar subdwarf gap' for mid L to early T types.

The remarkable outburst of the highly-evolved post period-minimum dwarf nova SSS J122221.7−311525

We report extensive 3-yr multiwavelength observations of the WZ Sge-type dwarf nova SSS J122221.7-311525 during its unusual double superoutburst, the following decline and in quiescence. The second segment of the superoutburst had a long duration of 33 d and a very gentle decline with a rate of 0.02 mag/d, and it displayed an extended post-outburst decline lasting at least 500 d. Simultaneously with the start of the rapid fading from the superoutburst plateau, the system showed the appearance of a strong near-infrared excess resulting in very red colours, which reached extreme values (B-I~1.4) about 20 d later. The colours then became bluer again, but it took at least 250 d to acquire a stable level. Superhumps were clearly visible in the light curve from our very first time-resolved observations until at least 420 d after the rapid fading from the superoutburst. The spectroscopic and photometric data revealed an orbital period of 109.80 min and a fractional superhump period excess <0.8 per cent, indicating a very low mass ratio q<0.045. With such a small mass ratio the donor mass should be below the hydrogen-burning minimum mass limit. The observed infrared flux in quiescence is indeed much lower than is expected from a cataclysmic variable with a near-main-sequence donor star. This strongly suggests a brown-dwarf-like nature for the donor and that SSS J122221.7-311525 has already evolved away from the period minimum towards longer periods, with the donor now extremely dim.

WISE J072003.20-084651.2: AN OLD AND ACTIVE M9.5 + T5 SPECTRAL BINARY 6 pc FROM THE SUN

[Abridged] We report observations of the recently discovered, nearby late-M dwarf WISE J072003.20-084651.2. Astrometric measurements obtained with TRAPPIST improve the distance measurement to 6.0$\pm$1.0 pc and confirm the low tangential velocity (3.5$\pm$0.6 km/s) reported by Scholz. Low-resolution optical spectroscopy indicates a spectral type of M9.5 and prominent H$\alpha$ emission (<LH$\alpha$/Lbol> = -4.68$\pm$0.06), but no evidence of subsolar metallicity or Li I absorption. Near-infrared spectroscopy reveals subtle peculiarities indicating the presence of a T5 binary companion, and high-resolution laser guide star adaptive optics imaging reveals a faint ($\Delta$H = 4.1) candidate source 0"14 (0.8 AU) from the primary. We measure a stable radial velocity of +83.8$\pm$0.3 km/s, indicative of old disk kinematics and consistent with the angular separation of the possible companion. We measure a projected rotational velocity of v sin i = 8.0$\pm$0.5 km/s, and find evidence of low-level variability (~1.5%) in a 13-day TRAPPIST lightcurve, but cannot robustly constrain the rotational period. We also observe episodic changes in brightness (1-2%) and occasional flare bursts (4-8%) with a 0.8% duty cycle, and order-of-magnitude variations in H$\alpha$ line strength. Combined, these observations reveal WISE J0720-0846 to be an old, very low-mass binary whose components straddle the hydrogen burning minimum mass, and whose primary is a relatively rapid rotator and magnetically active. It is one of only two known binaries among late M dwarfs within 10 pc of the Sun, both harboring a mid T-type brown dwarf companion. While this specific configuration is rare (1.4% probability), roughly 25% of binary companions to late-type M dwarfs in the local population are likely low-temperature T or Y brown dwarfs.

A detailed study of the main sequence of the globular cluster NGC 6397: can we derive constraints on the existence of multiple populations?

<i>Context. <i/>Globular clusters can no longer be regarded as examples of “simple stellar populations” as all those so far examined contain an important fraction of “second generation” stars, in which the light elements are processed through the hot CNO cycle, and helium variations may be present. Clusters apparently “simple” contain a majority of second generation stars.<i>Aims. <i/>If NGC 6397 contains a large fraction of “second generation” stars (<i>><i/>70% according to recent analysis), the helium abundance of its stars might also be affected, show some star-to-star variation, and be larger than the standard Big Bang abundance . Can we derive constraints on this issue from the analysis of the main sequence width and from its luminosity function?<i>Methods. <i/>We build up new models for the turnoff masses and the main sequence down to the hydrogen burning minimum mass, adopting two versions of an updated equation of state (EOS) including the OPAL EOS. Models consider different initial helium and CNO abundances to cover the range of possible variations between the first and second generation stars. We compare the models with the observational main sequence. We also make simulations of the theoretical luminosity function, for different choices of the mass function and of the mixture of first and second generation stars and compare them with the observed luminosity function by means of the Kolmogorov Smirnov – KS-test. <i>Results. <i/>The new models for very low mass stars compare well with previous models and show that the OPAL EOS is a good description in all the region of temperature and densities of very low mass stars for which it is computable. The analysis of the main sequence width shows that any helium variation must be confined within ~ 0.02 in the case of a CNO increase as suggested by literature, and we discuss the consequent implications for the model of self-enrichment. We also show that the KS test on the luminosity functions allows us to derive a best distance modulus for each age. For a population all made by stars with standard helium <i>Y=0.24<i/>, standard CNO abundances, and an age of 12 Gyr, choosing a double power law mass function for <i>M<i/> <i>><i/> 0.18 and for <i>M<0.18<i/> , the resulting theoretical luminosity function agrees well with the observed one (KS ~ 0.75 for a distance modulus = 12.31 <i>±<i/> 0.05 mag). Using non-standard CNO abundance for all the stars or for a fraction of 70%, the KS test provides comparable agreement (~KS <i>><i/> 0.55) with the observed luminosity function.<i>Conclusions. <i/>The study of the width of the main sequence at a different interval of magnitude is consistent with the hypothesis that both generations are present in the cluster. If the CNO increase suggested by spectroscopic observation is taken into account the small helium spread of the main sequence in NGC 6397 implies a substantial helium uniformity ( ~ 0.02) between first and second generation stars. The possible spread in helium doubles if an higher larger increase of CNO is considered. The luminosity function is in any case well consistent with the observed data.

THE BENCHMARK ULTRACOOL SUBDWARF HD 114762B: A TEST OF LOW-METALLICITY ATMOSPHERIC AND EVOLUTIONARY MODELS

We present a near-infrared spectroscopic study of HD 114762B, the latest-type metal-poor companion discovered to date and the only ultracool subdwarf with a known metallicity, inferred from the primary star to be [Fe/H] = -0.7. We obtained a medium-resolution Keck/OSIRIS J-band spectrum and a low-resolution IRTF/SpeX 0.8-2.4 um spectrum of HD 114762B. HD 114762B exhibits spectral features common to both late-type dwarfs and subdwarfs, and we assign it a spectral type of d/sdM9 +/- 1. We use a Monte Carlo technique to fit PHOENIX/GAIA synthetic spectra to the observations, accounting for the coarsely-gridded nature of the models. Fits to the entire OSIRIS J-band and to the metal-sensitive J-band atomic absorption features (Fe I, K I, and Al I lines) yield model parameters that are most consistent with the metallicity of the primary star and the high surface gravity expected of old late-type objects. The effective temperatures and radii inferred from the model atmosphere fitting broadly agree with those predicted by the evolutionary models of Chabrier & Baraffe, and the model color-absolute magnitude relations accurately predict the metallicity of HD 114762B. We conclude that current low-mass, mildly metal-poor atmospheric and evolutionary models are mutually consistent for spectral fits to medium-resolution J-band spectra of HD 114762B, but are inconsistent for fits to low-resolution near-infrared spectra of mild subdwarfs. Finally, we develop a technique for estimating distances to ultracool subdwarfs based on a single near-infrared spectrum. We show that this "spectroscopic parallax" method enables distance estimates accurate to < 10% of parallactic distances for ultracool subdwarfs near the hydrogen burning minimum mass. (abridged)

Discovery of a Second L Subdwarf in the Two Micron All Sky Survey

I report the discovery of the second L subdwarf identified in the Two Micron All Sky Survey, 2MASS J162620343925190. This high proper motion object ( yr 1 ) exhibits near-infrared spectral m p 1 .27 0 .03 features indicative of a subsolar metallicity L dwarf, including strong metal hydride and H 2O absorption bands, pressure-broadened alkali lines, and blue near-infrared colors caused by enhanced collision-induced H 2 absorption. This object is of later type than any of the known M subdwarfs but does not appear to be as cool as the apparently late-type sdL 2MASS 05328246. The radial velocity ( km s 1 ) and estimated tangential V p 260 35 rad velocity ( km s 1 ) of 2MASS 16263925 indicate membership in the Galactic halo, and this source V ≈ 90–210 tan is likely near or below the hydrogen-burning minimum mass for a metal-poor star. L subdwarfs such as 2MASS 16263925 are useful probes of gas and condensate chemistry in low-temperature stellar and brown dwarf atmospheres, but more examples are needed to study these objects as a population as well as to define a rigorous classification scheme.

Mass-Luminosity Relationship and Lithium Depletion for Very Low Mass Stars

We derive mass-luminosity relationships for very low mass stars (0.06 < m/M☉ < 0.6) for different metallicities. Calculations are conducted with the different nongray atmosphere models presently available to illustrate the uncertainties in the stellar models. The theoretical mass-magnitude relation reproduces accurately the observed relationship in the V band (Henry & McCarthy) but still underestimates the flux in the K band by ~0.5 mag, a consequence of the still inaccurate water opacities. Calculations based on the gray approximation are shown to yield incorrect results over the entire mass range of interest. We also show that the fate of objects near the hydrogen-burning limit depends significantly on the treatment of the atmosphere. Nongray effects yield cooler and less luminous objects for a given mass near the stellar/substellar transition, and thus a lower hydrogen-burning minimum mass, m ≈ 0.07 M☉, for solar metallicity.

Evolution of low-mass stars. implications for the dark matter distribution in the halo

We present up-to-date evolutionary models of low-mass stars, from M≤0.6 M⊙ down to the hydrogen burning minimum mass, using recent equation of state and synthetic spectra calculations. Comparison is made with observed luminosity function for these objects. We also present implications for the dark-matter distribution in the galactic halo.

New Evolutionary Tracks for Very Low Mass Stars

view Abstract Citations (133) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New Evolutionary Tracks for Very Low Mass Stars Baraffe, I. ; Chabrier, G. ; Allard, F. ; Hauschildt, P. H. Abstract We present new evolutionary calculations for low-mass and very low mass M dwarfs, for a metallicity range -2 ≤ [M/H] ≤ 0, down to the hydrogen-burning minimum mass (0.07 < M/Msun < 0.6). We use the most recent atmosphere models calculated by Allard & Hauschildt (1995), based on synthetic spectra at finite metallicity, and gray atmosphere models based on Alexander & Ferguson (1994) Rosseland opacities. Comparisons are made with observational results down to the bottom of the main sequence, for different metallicities, in magnitude-color and color-color diagrams. We find excellent agreement between theory and observations over the whole characteristic temperature/luminosity range. This enables us to determine the mass of the faintest objects observed, which is found to be mlim ≍ 0.085 Msun for [M/H] = 0 and -0.5, and mlim = 0.09 Msun for [M/H] = -1.5, for an age of 10 Gyr. We also examine the effect of the age, the metallicity, and the outer boundary conditions on the evolution. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/187924 Bibcode: 1995ApJ...446L..35B Keywords: STARS: LOW-MASS; BROWN DWARFS; STARS: EVOLUTION; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (2)

Evolution of very low mass stars and brown dwarfs. II - The population II

view Abstract Citations (65) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Very Low Mass Stars and Brown Dwarfs. II. The Population II D'Antona, Francesca Abstract This paper addresses a number of problems connected with the evolution of low-mass (M ≤ 0.8 M_sun;) and very low mass (M ≥ 0.07 M_sun;) stars having metal contents much smaller than the solar metal content. Two main chemical compositions bracketing the Population II regime are considered: helium and metal contents Y = 0.23, Z = 10-4 (Cox and Stewart opacities) and Y = 0.25, Z = 10-3 (Alexander, Johnson, and Rypma opacities). For these compositions, the hydrogen burning minimum mass is evaluated. It is confirmed that the atmospheric opacities are the main factor influencing the minimum main-sequence luminosity. Population II luminosity functions (LFs) are constructed and applied to the interpretation of globular cluster main-sequence LFs. Publication: The Astrophysical Journal Pub Date: September 1987 DOI: 10.1086/165582 Bibcode: 1987ApJ...320..653D Keywords: Brown Dwarf Stars; Globular Clusters; Opacity; Stellar Evolution; Stellar Luminosity; Stellar Mass; Main Sequence Stars; Mass To Light Ratios; Astrophysics; CLUSTERS: GLOBULAR; LUMINOSITY FUNCTION; OPACITIES; STARS: EVOLUTION; STARS: INTERIORS; STARS: POPULATION II full text sources ADS | data products SIMBAD (2) Related Materials (1) Part 1: 1985ApJ...296..502D