Characterization of stellar and sub-stellar atmospheres

In this work I aim characterize a number of (sub)-stellar companions. When studying isolated brown dwarfs, very low-mass stars, and directly imaged exoplanets with insignificant orbital motion, we have to rely on theoretical models to determine basic parameters such as mass, age, and consequently, effective temperature and surface gravity. While stellar and atmospheric models are rapidly evolving, we need a powerful tool to test and calibrate them. One way to test models is to compare theoretical isochrones produced by interior and atmospheric models with observed sequences in open clusters. Open clusters contain many objects of the same chemical composition and age, and spanning a range of masses. Using our own Lucky imaging observations and literature data, I constructed a single-star sequence for the Hyades open cluster. I compare the obtained sequence to a set of theoretical isochrones identifying systematic offsets and revealing probable issues in the models. However, there are many cases when it is impossible to test models before applying them to observations. One example is applying atmospheric models for constraining parameters of the coolest known Y dwarf WISE 0855-07. I demonstrate the limits of constraining effective temperature and the presence/absence of water clouds that are introduced by unknown systematic effects in models and observations. In the final chapter I introduce a novel method to take into account the above-mentioned systematics. I construct a ”systematics vector” that allows us to reveal problematic wavelength ranges when fitting atmospheric models to observed near-infrared spectra of brown dwarfs and directly imaged exoplanets. This approach plays a crucial role when retrieving abundances of brown dwarfs and exoplanets, in particularly, a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and exoplanets. I show the way of constraining this parameter while eliminating systematics effects, which significantly improves the reliability of a final result and our conclusions about formation history of certain exoplanets and brown dwarfs

Single stars in the Hyades open cluster. Fiducial sequence for testing stellar and atmospheric models

Age and mass determinations for isolated stellar objects remain model-dependent. While stellar interior and atmospheric theoretical models are rapidly evolving, we need a powerful tool to test them. Open clusters are good candidates for this role. We complement previous studies on the Hyades multiplicity by Lucky Imaging observations with the AstraLux Norte camera. This allows us to exclude possible binary and multiple systems with companions outside 2--7 AU separation and to create a "single-star sequence" for the Hyades. The sequence encompasses 250 main-sequence stars ranging from A5V to M6V. Using the "Tool for Astrophysical Data Analysis" (TA-DA), we create various theoretical isochrones applying different combinations of interior and atmospheric models. We compare the isochrones with the observed Hyades single-star sequence on J vs. J - K_s, J vs. J - H and K_s vs. H - K_s color-magnitude diagrams. As a reference we also compute absolute fluxes and magnitudes for all stars from X-ray to mid-infrared based on photometric measurements available in the literature(ROSAT X-ray, GALEX UV, APASS gri, 2MASS JHK_s, and WISE W1 to W).We find that combinations of both PISA and DARTMOUTH stellar interior models with BT-Settl 2010 atmospheric models describe the observed sequence well. The full sequence covers the mass range 0.13 to 2.3 Msun, and effective temperatures between 3060 K and 8200 K. Within the measurement uncertainties, the current generation of models agree well with the single-star sequence. The primary limitations are the uncertainties in the measurement of the distance to individual Hyades members, and uncertainties in the photometry. Additionally, a small (~0.05 mag) systematic offset can be noted on J vs. J - K and K vs. H - K diagrams - the observed sequence is shifted to redder colors from the theoretical predictions.Comment: 6 pages, 2 figures, 1 table. The extended version of the table will be available online soon. Accepted for publication in Astronomy & Astrophysic

Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57-531906.1AB

We present two epochs of MPG/ESO 2.2m GROND simultaneous 6-band ($r'i'z'JHK$) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57-531906.1AB. We report here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hours of focused observations on the night of UT 2013-04-22, as well as 4 hours of defocused (unresolved) observations on the night of UT 2013-04-16. We note a number of robust trends in our light curves. The $r'$ and $i'$ light curves appear to be anticorrelated with $z'$ and $H$ for the T0.5 component and in the unresolved lightcurve. In the defocused dataset, $J$ appears correlated with $z'$ and $H$ and anticorrelated with $r'$ and $i'$, while in the focused dataset we measure no variability for $J$ at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component lightcurve, the $K$ band lightcurve displays a significant phase offset relative to both $H$ and $z'$. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from 1D atmospheric models. We also report low-amplitude variability in $i'$ and $z'$ intrinsic to the L7.5 component.Comment: 14 pages, 5 figures, accepted to ApJ Letter

Deep z-band observations of the coolest y dwarf

Taisiya G. Kopytova, et al., “Deep z-band observations of the coolest y dwarf”, The Astrophysical Journal, Vol. 797(1), November 2014. © 2014. The American Astronomical Society.WISE J085510.83-071442.5 (hereafter, WISE 0855-07) is the coolest Y dwarf known to date and is located at a distance of 2.31 ± 0.08 pc, giving it the fourth largest parallax of any known star or brown dwarf system. We report deep z-band observations of WISE 0855-07 using FORS2 on UT1/Very Large Telescope. We do not detect any counterpart to WISE 0855-07 in our z-band images and estimate a brightness upper limit of AB mag > 24.8 (F ν <0.45 μJy) at 910 ± 65 nm with 3σ confidence. We combine our z-band upper limit with previous near- and mid-infrared photometry to place constraints on the atmospheric properties of WISE 0855-07 via comparison to models which implement water clouds in the atmospheres of T eff <300 K substellar objects. We find that none of the available models that implement water clouds can completely reproduce the observed spectral energy distribution of WISE 0855-07. Every model significantly disagrees with the (3.6 μm/4.5 μm) flux ratio and at least one other bandpass. Since methane is predicted to be the dominant absorber at 3-4 μm, these mismatches might point to an incorrect or incomplete treatment of methane in current models. We conclude that (a) WISE0855-07 has T eff 200-250 K, (b)Peer reviewe

Variability of CHXR 20: accretion, extinction, spots or a companion?

We study the photometric variability of a pre-main sequence star (K6, 0.9M⊙) CHXR 20. We test several scenarios for the variability including variable accretion, variable extinction, cool and hot spots on the stellar surface and the presence of a potential companion

Variability of CHXR 20: accretion, extinction, spots or a companion?

We study the photometric variability of a pre-main sequence star (K6, 0.9M⊙) CHXR 20. We test several scenarios for the variability including variable accretion, variable extinction, cool and hot spots on the stellar surface and the presence of a potential companion

WEATHER ON THE NEAREST BROWN DWARFS: RESOLVED SIMULTANEOUS MULTI-WAVELENGTH VARIABILITY MONITORING OF WISE J104915.57–531906.1AB

We present two epochs of MPG/ESO 2.2 m GROND simultaneous six-band (r'i'z' JHK) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57−531906.1AB. We report here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hr of focused observations on the night of 2013 April 22 (UT), as well as 4 hr of defocused (unresolved) observations on the night of 2013 April 16 (UT). We note a number of robust trends in our light curves. The r' and i' light curves appear to be anti-correlated with z' and H for the T0.5 component and in the unresolved light curve. In the defocused dataset, J appears correlated with z' and H and anti-correlated with r' and i', while in the focused dataset we measure no variability for J at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component light curve, the K band light curve displays a significant phase offset relative to both H and z'. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from one-dimensional atmospheric models. We also report low-amplitude variability in i' and z' intrinsic to the L7.5 component