Pulsating young brown dwarfs

We present the results of a nonadiabatic, linear stability analysis of models of very low-mass stars (VLMSs) and brown dwarfs (BDs) during the deuterium burning phase in the center. We find unstable fundamental modes with periods varying between ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 Msun BD. The growth time of the instability decreases with decreasing mass and remains well below the deuterium burning time scale in the mass range considered (0.1--0.02 Msun). These results are robust against variations of the relevant input physics in the evolutionary models. We identify possible candidates for pulsational variability among known VLMSs and BDs in nearby star forming regions whose location in the HR diagram falls within or close to the boundary of the instability strip. Finally, we discuss the possibility that the variability observed in a few objects with periods of ~1 hr can be interpreted in terms of pulsation.Comment: 5 pages, 3 figures, A&A Letters (in press

Significant uncertainties from calibrating overshooting with eclipsing binary systems

The precise measurement of the masses and radii of stars in eclipsing binary systems provides a window into uncertain processes in stellar evolution, especially mixing at convective boundaries. Recently, these data have been used to calibrate models of convective overshooting in the cores of main sequence stars. In this study we have used a small representative sample of eclipsing binary stars with $1.25 \leq M/\text{M}_\odot < 4.2$ to test how precisely this method can constrain the overshooting and whether the data support a universal stellar mass--overshooting relation. We do not recover the previously reported stellar mass dependence for the extent of overshooting and in each case we find there is a substantial amount of uncertainty, that is, the same binary pair can be matched by models with different amounts of overshooting. Models with a moderate overshooting parameter $0.013 \leq f_\text{os} \leq 0.014$ (using the scheme from Herwig et al. 1997) are consistent with all eight systems studied. Generally, a much larger range of $f_\text{os}$ is suitable for individual systems. In the case of main sequence and early post-main sequence stars, large changes in the amount of overshooting have little effect on the radius and effective temperature, and therefore the method is of extremely limited utility.Comment: Accepted for publication in A&

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.Comment: 13 pages, Latex file, uses aasms4.sty, accepted for publication in ApJ Letter

The Impact of Transiting Planet Science on the Next Generation of Direct-Imaging Planet Searches

Within the next five years, a number of direct-imaging planet search instruments, like the VLT SPHERE instrument, will be coming online. To successfully carry out their programs, these instruments will rely heavily on a-priori information on planet composition, atmosphere, and evolution. Transiting planet surveys, while covering a different semi-major axis regime, have the potential to provide critical foundations for these next-generation surveys. For example, improved information on planetary evolutionary tracks may significantly impact the insights that can be drawn from direct-imaging statistical data. Other high-impact results from transiting planet science include information on mass-to-radius relationships as well as atmospheric absorption bands. The marriage of transiting planet and direct-imaging results may eventually give us the first complete picture of planet migration, multiplicity, and general evolution.Comment: 4 pages, 3 figures, IAU Transiting Planets Proceedings, in pres

Adaptive Optics Observations of Exoplanets, Brown Dwarfs, & Binary Stars

The current direct observations of brown dwarfs and exoplanets have been obtained using instruments not specifically designed for overcoming the large contrast ratio between the host star and any wide-separation faint companions. However, we are about to witness the birth of several new dedicated observing platforms specifically geared towards high contrast imaging of these objects. The Gemini Planet Imager, VLT-SPHERE, Subaru HiCIAO, and Project 1640 at the Palomar 5m telescope will return images of numerous exoplanets and brown dwarfs over hundreds of observing nights in the next five years. Along with diffraction-limited coronagraphs and high-order adaptive optics, these instruments also will return spectral and polarimetric information on any discovered targets, giving clues to their atmospheric compositions and characteristics. Such spectral characterization will be key to forming a detailed theory of comparative exoplanetary science which will be widely applicable to both exoplanets and brown dwarfs. Further, the prevalence of aperture masking interferometry in the field of high contrast imaging is also allowing observers to sense massive, young planets at solar system scales (~3-30 AU)---separations out of reach to conventional direct imaging techniques. Such observations can provide snapshots at the earliest phases of planet formation---information essential for constraining formation mechanisms as well as evolutionary models of planetary mass companions. As a demonstration of the power of this technique, I briefly review recent aperture masking observations of the HR 8799 system. Moreover, all of the aforementioned techniques are already extremely adept at detecting low-mass stellar companions to their target stars, and I present some recent highlights.Comment: Invited Review for IAU Symposium 28

Latest news on the Physics of Brown dwarfs

The physics of brown dwarfs has continuously improved since the discovery of these astrophysical bodies. The first important developments were devoted to the description of their mechanical structure, with the derivation of an appropriate equation of state, and the modelling of their atmosphere characterised by strong molecular absorption. New challenges are arising with progress in observational techniques which provide data of unprecedented accuracy. The goal of this chapter is to describe some of the current challenges for the theory of brown dwarfs. Those challenges concerns atmospheric dust and cloud, non-equilibrium atmospheric chemistry, the effect of rotation and magnetic fields on internal structure and the very early phases of evolution characterised by accretion processes. The field remains lively as more and more high quality observational data become available and because of increasing discoveries of exoplanets. Indeed, many physical properties of giant exoplanets can be described by the same theory as brown dwarfs, as described in this chapter.Comment: Invited Chapter - "50 Years of Brown Dwarfs", ed. Vicki Joergens, Astrophysics and Space Science (final version available on http://link.springer.com/book/10.1007/978-3-319-01162-2

Substructure in Tidal Streams; Tributaries in the Anticenter Ring

We report on the detection in Sloan Digital Sky Survey data of at least three, roughly parallel components in a 65 degree-long stellar stream complex previously identified with the Anticenter or Monoceros Ring. The three-stream complex varies in width from 4 to 6 degrees along its length and appears to be made up of two or more narrow substreams as well as a broader, diffuse component. The width and complexity of the stream indicate that the progenitor was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc distant and is oriented almost perpendicularly to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies and a preliminary orbit does not point to any viable progenitor candidates. Orbits for the narrower substreams can be modeled with velocity offsets from the broad component of about 8 km/s. We suggest that the broad component is likely to be the remains of a dwarf galaxy, while the narrower streams constitute the remnants of dynamically distinct components which may have included a native population of globular clusters. While the color of the main sequence turn-off is not unlike that for the Monoceros Ring, neither the visible stream nor any reasonable projection of its orbit passes through Monoceros or Canis Major, and we conclude that this stream is probably unrelated to the overdensities found in these regions.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter

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.Comment: 9 pages, Latex file, uses mn.sty, accepted for publication in MNRA