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I. Baraffe

Publications and source records attributed to I. Baraffe.

At least 91 records · Page 5Linked to original sources
arXiv2003

Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 20945

We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models reproduce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradiation effects and taking into account the extension of the outer atmospheric layers, we still find $\sim$ 20% discrepancy between observed and theoretical radii. An extra source of energy seems to be required to explain the observed value of the first transit planet.

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arXiv2002

QZ Serpentis: A Dwarf Nova with a 2-Hour Orbital Period and an Anomalously Hot, Bright Secondary Star

We present spectroscopy and time-series photometry of the dwarf nova QZ Ser. The spectrum shows a rich absorption line spectrum of type K4 +- 2. K-type secondary stars are generally seen in dwarf novae with orbital periods P-orb around 6 h, but in QZ Ser the absorption radial velocities show an obvious modulation (semi-amplitude 207(5) km/s) at P-orb = 119.752(2) min, much shorter than typical for such a relatively warm and prominent secondary spectrum. The H-alpha emission-line velocity is modulated at the same period and roughly opposite phase. Time-series photometry shows flickering superposed on a modulation with two humps per orbit, consistent with ellipsoidal variation of the secondary's light. QZ Ser is a second example of a relatively short-period dwarf nova with a surprisingly warm secondary. Model calculations suggest that the secondary is strongly enhanced in helium, and had already undergone significant nuclear evolution when mass transfer began. Several sodium absorption features in the secondary spectrum are unusually strong, which may indicate that the present-day surface was the site of CNO-cycle hydrogen burning in the past.

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arXiv2002

Distortion of secondaries in semi-detached binaries and the cataclysmic variable period minimum

Based on SPH simulations, we quantify the geometrical distortion effect due to tidal and rotational forces on polytropic secondaries in semi-detached binaries. The main effect is an expansion of the polytropic star, with an effect on the radius of $\sim$ 5%-12%, depending on the polytropic index and the mass ratio. We apply such distortion effects to the secular evolution of secondaries in cataclysmic variable systems. We focus on systems below the 2-3h period gap and that approach the minimum period. We find a significant increase of the predicted minimum period ($\sim$ 4% if changes in the secondary's thermal relaxation are approximately taken into account). Though an improvement, the effect is not big enough to solve the mismatch between predicted and observed minimum period at 80 min.

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arXiv2001

Evolution and Explosion of Very Massive Primordial Stars

While the modern stellar IMF shows a rapid decline with increasing mass, theoretical investigations suggest that very massive stars (>100 solar masses) may have been abundant in the early universe. Other calculations also indicate that, lacking metals, these same stars reach their late evolutionary stages without appreciable mass loss. After central helium burning, they encounter the electron-positron pair instability, collapse, and burn oxygen and silicon explosively. If sufficient energy is released by the burning, these stars explode as brilliant supernovae with energies up to 100 times that of an ordinary core collapse supernova. They also eject up to 50 solar masses of radioactive Ni56. Stars less massive than 140 solar masses or more massive than 260 solar masses should collapse into black holes instead of exploding, thus bounding the pair-creation supernovae with regions of stellar mass that are nucleosynthetically sterile. Pair-instability supernovae might be detectable in the near infrared out to redshifts of 20 or more and their ashes should leave a distinctive nucleosynthetic pattern.

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arXiv2001

Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages

We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses $m \le 1.4 \msol$ based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages $\simle$ 1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravity and $\te < 4000$ K, whereas it vanishes as gravity increases. This effect adds another source of uncertainty on evolutionary tracks at very early phases. We show that at low surface gravity ($\log g \simle 3.5$,) the common picture of vertical Hayashi lines with constant $\te$ is oversimplified. The effect of a variation of initial deuterium abundance is studied. We compare our models with evolutionary tracks available in the literature and discuss the main differences. We finally analyse to which extent current observations of young systems provide a good test for pre-Main Sequence tracks.

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arXiv2001

The Substellar Mass Function in sigma Orionis

We combine results from imaging searches for substellar objects in the sigma Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin^2. We identify 64 very low-mass cluster member candidates in this region. We have available three color (IZJ) photometry for all of them, spectra for 9 objects, and K photometry for 27% of our sample. These data provide a well defined sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the cluster is affected by little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to 0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013 Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in sigma Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 Msol should be abundant in the solar vicinity, with a density similar to M-type stars.

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arXiv2001

Period - magnitude relationships in BVIJHK-Bands for fundamental mode and first overtone Cepheids

We present theoretical period - magnitude relationships for Cepheids in different filters for fundamental and first overtone pulsators, completing the work by Alibert et al. (1999). The results are provided for different metallicities characteristic of the Magellanic Clouds and the Milky Way. In contrast to the fundamental mode, we find a small metallicity effect on the period - luminosity relationship for the first overtone, due to the sensitivity of the period ratio $P_1$/$P_0$ with metallicity. Comparison is made with observations from OGLE and EROS in the Small and Large Magellanic Clouds. We emphasize the constraint on theoretical predictions provided by the combination of both fundamental and first overtone observed sequences. We obtain excellent agreement between models and data in a $\log P$ - $\wi$ (Wesenheit index) diagram for a distance modulus for the LMC $μ_0$ = 18.60 - 18.70. We analyse the uncertainties of the fundamental period - magnitude relationships and the consequences on distance determination. We show that an arbitrary shift of the instability strip by 350 K in $\te$ yields up to 0.45 mag effect on $\mv$ at a given period, whereas the effect is less than 0.1 mag in the $K$-band. Using recent near-IR observations in the Large Magellanic Cloud and our $P$ - $\mk$ relationship, we derive a distance modulus for the LMC in agreement with the value based on $\wi$ data.

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arXiv2000

On the stability of very massive primordial stars

The stability of metal-free very massive stars ($Z$ = 0; $M = 120 - 500 \msol$) is analyzed and compared with metal-enriched stars. Such zero-metal stars are unstable to nuclear-powered radial pulsations on the main sequence, but the growth time scale for these instabilities is much longer than for their metal-rich counterparts. Since they stabilize quickly after evolving off the ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss in Z = 0 stars. For reasonable assumptions regarding the efficiency of converting pulsational energy into mass loss, we find that, even for the larger masses considered, the star may die without losing a large fraction of its mass. We find a transition between the $ε$- and $κ$-mechanisms for pulsational instability at $Z\sim 2\E{-4} - 2\E{-3}$. For the most metal-rich stars, the $κ$-mechanism yields much shorter $e$-folding times, indicating the presence of a strong instability. We thus stress the fundamental difference of the stability and late stages of evolution between very massive stars born in the early universe and those that might be born today.

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arXiv2000

Evolution and Nucleosynthesis of Very Massive Primordial Stars

We investigate the evolution, final fate, and nucleosynthetic yields of rotating and non-rotating very massive stars (VMS) of zero metallicity. First we address the issue of mass loss during hydrogen burning due to vibrational instabilities. We find that these objects are much more stable than what was found in previous studies of VMS of solar composition, and expect only negligible mass loss driven by the pulsations. As these stars thus reach the end of their evolution with massive helium cores, they encounter the pair-creation instability. We find that for helium core masses of ~64...133 solar masses these stars are completely disrupted with explosion energies of up to ~1E53 erg and eject up to ~60 solar masses of Ni56 Stars with more massive helium cores collapse into black holes. We present the first calculations that follow the collapse of such a massive rotating star and predict that X-ray burst and significant gravitational wave emission could result.

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arXiv2000

Mass estimates in short-period compact binaries

Using stellar models we investigate the relation between mass and the spectral type of the secondary star in low-mass short-period compact binaries such as cataclysmic variables and soft X-ray transients. Allowing for different mass transfer rates and different system ages prior to mass transfer we find that the secondaries should populate a band in the spectral type vs. mass plane. The mass M_ms of a ZAMS star with the same spectral type as the donor is effectively an upper limit to the donor mass. The lower mass limit for a given spectral type depends on the mixing length parameter. If this is large, there is no lower limit if the spectral type is later than K6. The band width decreases from 0.4 M_sun at K6 to less than 0.2 M_sun at K0. If the mixing length parameter is small, there is no lower mass limit if the spectral type is later than M2, and the band width decreases from 0.2 M_sun at M2 to less than 0.1 M_sun for types earlier than K0. We also point out an error in the method suggested by Beekman et al. (1997) to estimate the primary mass in a soft X-ray transient.

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arXiv2000

Deuterium-burning in substellar objects

We consider the depletion of primordial deuterium in the interior of substellar objects as a function of mass, age and absolute magnitude in several photometric passbands. We characterize potential spectroscopic signatures of deuterium in the lines of deuterated water HDO. These results will serve as a useful, independent diagnostic to characterize the mass and/or the age of young substellar objects, and to provide an independent age determination of very young clusters. These results can serve to identify objects at the deuterium-burning limit and to confront the theoretical prediction that D-burning is a necessary condition to form star-like objects.

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arXiv2000

Pre-Main Sequence models for low-mass stars and brown dwarfs

We present evolutionary models for low mass stars and brown dwarfs ($m \le 1.2 \msol$) based on recent improvement of the theory: equation of state, atmosphere models, ... We concentrate on early evolutionary phases from the initial deuterium burning phase to the zero-age Main Sequence. Evolutionary models for young brown dwarfs are also presented. We discuss the uncertainties of the present models. We analyse the difficulties arising when comparing models with observations for very young objects, in particular concerning the problem of reddening.

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arXiv2000

Theory of Low-Mass Stars and Substellar Objects

Since the discovery of the first bona-fide brown dwarfs and extra-solar planets in 1995, the field of low mass stars and substellar objects has considerably progressed, both from theoretical and observational viewpoints.Recent developments in the physics entering the modeling of these objects have led to significant improvements in the theory and to a better understanding of their mechanical and thermal properties. This theory can now be confronted with observations directly in various observational diagrams (color-color, color-magnitude, mass-magnitude, mass-spectral type), a stringent and unavoidable constraint which became possible only recently, with the generation of synthetic spectra. In this paper, we present the current state-of-the-art general theory of low-mass stars and sub-stellar objects, from one solar mass to one Jupiter mass, regarding primarily their interior structure and evolution. This review is a natural complement to the previous review on the atmosphere of low-mass stars and brown dwarfs (Allard et al 1997). Special attention is devoted to the comparison of the theory with various available observations. The contribution of low-mass stellar and sub-stellar objects to the Galactic mass budget is also analysed.

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arXiv2000

Evolutionary models for very-low-mass stars and brown dwarfs with dusty atmospheres

We present evolutionary calculations for very-low-mass stars and brown dwarfs based on synthetic spectra and non-grey atmosphere models which include dust formation and opacity, i.e. objects with $\te\simle 2800$ K. The interior of the most massive brown dwarfs is shown to develop a conductive core after $\sim 2$ Gyr which slows down their cooling. Comparison is made in optical and infrared color-magnitude diagrams with recent late-M and L-dwarf observations. The saturation in optical colors and the very red near-infrared colors of these objects are well explained by the onset of dust formation in the atmosphere. Comparison of the faintest presently observed L-dwarfs with these dusty evolutionary models suggests that dynamical processes such as turbulent diffusion and gravitational settling are taking place near the photosphere. As the effective temperature decreases below $\te\approx 1300-1400$ K, the colors of these objects move to very blue near-infrared colors, a consequence of the ongoing methane absorption in the infrared. We suggest the possibility ofa brown dwarf dearth in $J,H,K$ color-magnitude diagrams around this temperature.

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arXiv2000

On the late spectral types of cataclysmic variable secondaries

We investigate why the spectral type of most cataclysmic variable (CV) secondaries is significantly later than that of a ZAMS star with the same mean density. Using improved stellar input physics, tested against observations of low-mass stars at the bottom of the main sequence, we calculate the secular evolution of CVs with low-mass donors. We consider sequences with different mass transfer rates and with a different degree of nuclear evolution of the donor prior to mass transfer. Systems near the upper edge of the gap ($P \sim 3 - 6$ h) can be reproduced by models with a wide range of mass transfer rates from $1.5 \times 10^{-9} \msolyr$ to $10^{-8} \msolyr$. Evolutionary sequences with a small transfer rate and donors that are substantially evolved off the ZAMS (central hydrogen content $0.05-0.5$) reproduce CVs with late spectral types above $P \simgr$ 6 h. Systems with the most discrepant (late) spectral type should have the smallest donor mass at any given $P$. Consistency with the period gap suggests that the mass transfer rate increases with decreasing donor mass for evolved sequences above the period gap. In this case, a single-parameter family of sequences with varying $\xc$ and increasing mass transfer rate reproduces the full range of observed spectral types. This would imply that CVs with such evolved secondaries dominate the CV population.

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arXiv1999

Barnes--Evans relations for late--type giants and dwarfs

The visual surface brightness of K/M giants and dwarfs with near-solar metallicity differ slightly in agreement with the gravity effects predicted by recent theoretical models. We show that M-dwarfs display also a metallicity dependence of the surface brightness in the infrared $K$-band in agreement with theory. Based on these results, we present improved Barnes-Evans type relations and estimate the radii of 60 single or presumed M and K-dwarfs

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arXiv1999

Period - luminosity - color - radius relationships of Cepheids as a function of metallicity: evolutionary effects

Based on consistent evolutionary and pulsation calculations, we analyse the effect of metallicity and of different convection treatments in the stellar models on period - magnitude, - color and - radius relationships. In order to perform an accurate comparison with observations, we have computed grids of atmosphere models and synthetic spectra for different metallicities, covering the range of effective temperatures and gravities relevant for Cepheids. The models are compared to recent observations of galactic and Magellanic Clouds Cepheids. Unprecedented level of agreement is found between models and observations. We show that within the range of metallicity for the Galaxy and the Magellanic Clouds, a change of slope in the period - luminosity (PL) relationship is predicted at low periods, due to the reduction of the blue loop during core He burning. The minimum mass undergoing a blue loop and consequently the critical period at which this change of slope occurs depend on the metallicity Z and on the convection treatment in the stellar models. However, besides this change of slope, we do not find any significant effect of metallicity on period - magnitude relationships from V to K bands, and on period - color relationships in IR colors. We only find a detectable effect of $Z$ on $(B-V)$ colors. These results are not affected by uncertainties inherent to current stellar models, mainly due to convection treatment.

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