scholarly journals What Do Young Brown Dwarfs Tell Us About Exoplanets?

2015 ◽  
Vol 10 (S314) ◽  
pp. 226-231
Author(s):  
Katelyn N. Allers ◽  
Michael C. Liu ◽  
Trent J. Dupuy
Keyword(s):  
Effective Temperature ◽  
Brown Dwarfs ◽  
Evolutionary Models ◽  
Atmospheric Models ◽  
Sky Surveys ◽  
Fundamental Properties ◽  
Dust Clouds ◽  
Planetary Mass ◽  
Substellar Objects ◽  
Successes And Challenges

AbstractIn recent years, all-sky surveys have uncovered a new and interesting population of young (≈10–200 Myr), nearby substellar objects. Many of these objects have inferred masses and temperatures that overlap those of directly imaged exoplanets. These young brown dwarfs provide valuable analogs to young, dusty exoplanets in a context where detailed spectroscopic observations across a broad range of wavelengths and at high S/N are possible. How do the temperatures inferred by atmospheric models and evolutionary models compare? Can we determine the formation mechanism of a young planetary-mass object? How well do we understand the role that disequilibrium chemistry and dust clouds play in the atmospheres of these objects? We review the successes and challenges in determining the fundamental properties (mass, log(g), effective temperature) of young substellar objects, both brown dwarfs and gas-giant exoplanets.

scholarly journals Dynamical masses for the nearest brown dwarf binary: ε Indi Ba,b

2009 ◽  
Vol 5 (H15) ◽  
pp. 761-761
Author(s):  
C. V. Cardoso ◽  
M. J. McCaughrean ◽  
R. R. King ◽  
L. M. Close ◽  
R.-D. Scholz ◽  
...  
Keyword(s):  
Main Sequence ◽  
Brown Dwarfs ◽  
Evolutionary Models ◽  
Atmospheric Models ◽  
Brown Dwarf ◽  
Near Ir ◽  
The Earth ◽  
Benchmark System ◽  
Model Independent ◽  
Dynamical Masses

Binary brown dwarfs are important because their dynamical masses can be determined in a model-independent way. If a main sequence star is also involved, the age and metallicity for the system can be determined, making it possible to break the sub-stellar mass-age degeneracy. The most suitable benchmark system for intermediate age T dwarfs is ε Indi Ba,b, two T dwarfs (spectral types T1 and T6; McCaughrean et al. (2004)) orbiting a K4.5V star, ε Indi A, at a projected separation of 1460AU. At a distance of 3.6224pc (HIPPARCOS distance to ε Indi A; van Leeuwen (2007)), these are the closest brown dwarfs to the Earth, and thus both components are bright and the system is well-resolved. The system has been monitored astrometrically with NACO and FORS2 on the VLT since June 2004 and August 2005, respectively, in order to determine the system and individual masses independent of evolutionary models. We have obtained a preliminary system mass of 121±1MJup. We have also analysed optical/near-IR spectra (0.6-5.0μm at a resolution up to R~5000; King et al. (2009)) allowing us to determine bolometric luminosities, compare and calibrate evolutionary and atmospheric models of T dwarfs at an age of 4-8Gyr.

scholarly journals Disks in Brown Dwarf Systems

2003 ◽  
Vol 211 ◽  
pp. 127-132
Author(s):  
Leonardo Testi ◽  
Antonella Natta ◽  
Fernando Comerón ◽  
Ernesto Oliva ◽  
Francesca D'Antona
Keyword(s):  
Infrared Spectroscopy ◽  
Near Infrared ◽  
Brown Dwarfs ◽  
Circumstellar Disks ◽  
Brown Dwarf ◽  
Infrared Excess ◽  
Mid Infrared ◽  
Star Forming ◽  
Planetary Mass ◽  
Substellar Objects

We discuss evidence for and properties of disks associated with brown dwarfs in the star-forming region ρ Oph. We derived photospheric parameters from low resolution near infrared spectroscopy and modeled the mid-infrared excess of nine substellar object candidates in the ρ OphISOCAM survey of Bontemps et al. (2001). In all cases, the mid-infrared excess is consistent with the SED expected from irradiated disks. These results suggest that circumstellar disks are commonly associated to young brown dwarfs and planetary-mass objects. Finally, we discuss the possibility of using these data to discriminate between various formation scenarios for substellar objects.

scholarly journals Photospheric Properties of L and T Dwarfs

2003 ◽  
Vol 211 ◽  
pp. 317-324
Author(s):  
S. K. Leggett ◽  
X. Fan ◽  
T. R. Geballe ◽  
D. A. Golimowski ◽  
G. R. Knapp
Keyword(s):  
Effective Temperature ◽  
Brown Dwarfs ◽  
Spectral Energy ◽  
Atmospheric Models ◽  
Mass Measurements ◽  
Energy Distributions ◽  
Effective Temperatures ◽  
Alkali Absorption ◽  
The Masses ◽  
Absorption Features

The photospheric spectra of all L and T dwarfs contain strong molecular bands and alkali absorption features, and those of L and early T dwarfs are also affected by dust. Although work on atmospheric models is addressing the treatment of dust, and is increasing the completeness of molecular opacity linelists, the effective temperatures of L and T dwarfs cannot be derived confidently from the modelled spectral energy distributions. However, because the radii of brown dwarfs older than 0.1 Gyr vary little with mass, measurements of intrinsic luminosity (i.e. total integrated flux and parallax) accurately determine effective temperature. Using this method we find a well constrained relationship between the effective temperatures of L and T dwarfs and spectral type, but with temperature nearly constant from L7 to T4. More work is required to determine the uniqueness of this relationship and to constrain the masses and ages of brown dwarfs.

scholarly journals The Remarkable Evolution of the Post-Agb Star FG SGE

1998 ◽  
Vol 11 (1) ◽  
pp. 363-363
Author(s):  
Johanna Jurcsik ◽  
Benjamin Montesinos
Keyword(s):  
Time Scales ◽  
Effective Temperature ◽  
Planetary Nebula ◽  
Planetary Nebulae ◽  
Central Star ◽  
Evolutionary Models ◽  
Single Star ◽  
Line Intensities ◽  
Evolutionary Changes ◽  
Evolutionary Studies

FG Sagittae is one of the most important key objects of post-AGB stellar evolutionary studies. As a consequence of a final helium shell flash, this unique variable has shown real evolutionary changes on human time scales during this century. The observational history was reviewed in comparison with predictions from evolutionary models. The central star of the old planetary nebula (Hel-5) evolved from left to right in the HR diagram, going in just hundred years from the hot region of exciting sources of planetary nebulae to the cool red supergiant domain just before our eyes becoming a newly-born post-AGB star. The effective temperature of the star was around 50,000 K at the beginning of this century, and the last estimates in the late 1980s give 5,000-6,500 K. Recent spectroscopic observations obtained by Ingemar Lundström show definite changes in the nebular line intensities. This fact undoubtedly rules out the possibility that, instead of FG Sge, a hidden hot object would be the true central star of the nebula. Consequently, the observed evolutionary changes are connected with the evolution of a single star.

scholarly journals A Deep Large area Search for Low Luminosity Stars

1998 ◽  
Vol 11 (1) ◽  
pp. 435-435
Author(s):  
Hugh R.A. Jones ◽  
Mike R.S. Hawkins
Keyword(s):  
Spectral Type ◽  
Low Temperatures ◽  
Current Model ◽  
Brown Dwarfs ◽  
Evolutionary Models ◽  
Spectral Indices ◽  
Late Type ◽  
Brown Dwarf ◽  
Large Area ◽  
M Dwarfs

In a recent survey for faint red stars from a digital stack of Schmidt plates a number of candidate objects were identified. Parallax’s for three of these objects have been reported showing them to have luminosities which interpreted within the available evolutionary models indicate them to be good brown dwarf candidates. Here we examine spectra of these objects and others from the plate stack. Using standard spectral indices we find that for a given spectral type their spectra are more consistent with the Pleiades brown dwarfs (PPL 15, Teide 1 and Calar 3) than with standard late-type M dwarfs. Our interpretation is that this is due to their selection by RF IN colours which at values > 3 preferentially selects objects with relatively low gravities. For late-type M dwarfs and brown dwarfs low gravities are expected to be a reliable indication of youth. We also notice that the stack objects generally have strong FeH absorption for their spectral type. Current model atmospheres suggest that FeH strongly increases in strength toward lower metallicities and lower temperatures. We believe that this is not consistent with the available observational evidence from late-type M dwarfs. It is possible that solid Fe is forming inthe low temperature atmospheres relatively depleting FeH strengths toward lower temperatures. We find some evidence that for dwarfs at low temperatures dust formation is less prevalent in lower gravity objects suggesting that dwarfs at low temperatures stronger FeH may be an indication of youth. In addition to the spectral evidence the three stack objects whose parallax’s have been measured show small tangential velocities which is a further indication of youth.

scholarly journals Water in Sunspots and Stars

2002 ◽  
Vol 12 ◽  
pp. 70-72 ◽  
Author(s):  
Peter F. Bernath
Keyword(s):  
Water Vapor ◽  
Brown Dwarfs ◽  
Substellar Objects ◽  
Cool Stars

AbstractWater is found in a wide variety of cool stars and substellar objects such as brown dwarfs. The detection of water vapor in such objects is reviewed.

scholarly journals 3D spectroscopic analysis of helium-line white dwarfs

2020 ◽  
Author(s):  
Elena Cukanovaite ◽  
Pier-Emmanuel Tremblay ◽  
Pierre Bergeron ◽  
Bernd Freytag ◽  
Hans-Günter Ludwig ◽  
...  
Keyword(s):  
Effective Temperature ◽  
White Dwarfs ◽  
Energy Transport ◽  
3D Models ◽  
Spectroscopic Technique ◽  
Surface Gravity ◽  
Atmospheric Models ◽  
Physical Treatment ◽  
Spectroscopic Parameters ◽  
1D Model

Abstract In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with −10.0 ≤ log (H/He) ≤−2.0, 7.5 ≤ log g ≤9.0 and 12 000 K ≲ Teff ≲ 34 000 K, based on 282 3D atmospheric models calculated with the CO5BOLD radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing SDSS sample of DB and DBA white dwarfs, we find significant corrections both for effective temperature and surface gravity. The 3D log g corrections are most significant for Teff ≲ 18, 000 K, reaching up to −0.20 dex at log g = 8.0. However, in this low effective temperature range, the surface gravity determined from the spectroscopic technique, can also be significantly affected by the treatment of the neutral van der Waals line broadening of helium and by non-ideal effects due to the perturbation of helium by neutral atoms. Thus, by removing uncertainties due to 1D convection, our work showcases the need for improved description of microphysics for DB and DBA model atmospheres. Overall, we find that our 3D spectroscopic parameters for the SDSS sample are generally in agreement with Gaia DR2 absolute fluxes within 1-3σ for individual white dwarfs. By comparing our results to DA white dwarfs, we determine that the precision and accuracy of DB/DBA atmospheric models are similar. For ease of user application of the correction functions, we provide an example Python code.

scholarly journals Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

2018 ◽  
Vol 620 ◽  
pp. A196 ◽  
Author(s):  
Leila M. Calcaferro ◽  
Alejandro H. Córsico ◽  
Leandro G. Althaus ◽  
Alejandra D. Romero ◽  
S. O. Kepler
Keyword(s):  
Stellar Evolution ◽  
Effective Temperature ◽  
Evolutionary Models ◽  
Long Period ◽  
Low Mass ◽  
Complete Set ◽  
Effective Temperatures ◽  
Binary Evolution ◽  
Stellar Masses ◽  
Gravity Mode

Context. Some low-mass white-dwarf (WD) stars with H atmospheres currently being detected in our galaxy, show long-period g(gravity)-mode pulsations, and comprise the class of pulsating WDs called extremely low-mass variable (ELMV) stars. At present, it is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of low-mass WDs with thin H envelopes is also possible. Aims. We present a thorough asteroseismological analysis of ELMV stars on the basis of a complete set of fully evolutionary models that represents low-mass He-core WD stars harboring a range of H envelope thicknesses. Although there are currently nine ELMVs, here we only focus on those that exhibit more than three periods and whose periods do not show significant uncertainties. Methods. We considered g-mode adiabatic pulsation periods for low-mass He-core WD models with stellar masses in the range [0.1554–0.4352] M⊙, effective temperatures in the range [6000–10 000] K, and H envelope thicknesses in the interval −5.8 ≲ log(MH/M⋆)≲ −1.7. We explore the effects of employing different H-envelope thicknesses on the adiabatic pulsation properties of low-mass He-core WD models, and perform period-to-period fits to ELMV stars to search for a representative asteroseismological model. Results. We found that the mode-trapping effects of g modes depend sensitively on the value of MH, with the trapping cycle and trapping amplitude larger for thinner H envelopes. We also found that the asymptotic period spacing, ΔΠa, is longer for thinner H envelopes. Finally, we found asteroseismological models (when possible) for the stars under analysis, characterized by canonical (thick) and by thin H envelope. The effective temperature and stellar mass of these models are in agreement with the spectroscopic determinations. Conclusions. The fact that we have found asteroseismological solutions with H envelopes thinner than canonical gives a suggestion of the possible scenario of formation of these stars. Indeed, in the light of our results, some of these stars could have been formed by binary evolution through unstable mass loss.

scholarly journals Spectroscopy of Candidate Members of the Sco-Cen Complex*

2021 ◽  
Vol 163 (1) ◽  
pp. 26
Author(s):  
K. L. Luhman ◽  
T. L. Esplin
Keyword(s):  
Brown Dwarfs ◽  
Evolutionary Models ◽  
Stellar Masses

Abstract We present spectroscopy of 285 previously identified candidate members of populations in the Sco-Cen complex, primarily Ophiuchus, Upper Sco, and Lupus. The spectra are used to measure spectral types and diagnostics of youth. We find that 269 candidates exhibit signatures of youth in our spectra or previous data, which is consistent with their membership in Sco-Cen. We have constructed compilations of candidate members of Ophiuchus, Upper Sco, and Lupus that have spectral classifications and evidence of youth, which contain a total of 2274 objects. In addition, we have used spectra from previous studies to classify three sources in Ophiuchus that have been proposed to be protostellar brown dwarfs: ISO Oph 70, 200, and 203. We measure spectral types of early M from those data, which are earlier than expected for young brown dwarfs based on evolutionary models (≳M6.5) and instead are indicative of stellar masses (∼0.6 M ⊙).

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