5,207 research outputs found
Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey
We present the results of a search for cool white dwarfs in the United
Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area
Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky
Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their
neutral optical colors and blue near-infrared colors, as well as faint Reduced
Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini
Observatory, and showed the majority of the candidates to be newly identified
cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants.
Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with
effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky
has produced thirteen new white dwarfs. Model fits to the photometry show that
seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and
cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s
and are likely to be thick disk 10-11 Gyr-old objects. The other half of the
sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr,
and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with
unusually high velocities, or lower-mass remnants of thick disk or halo late-F
or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures
and 5 Table
The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres
The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest
known brown dwarfs, with effective temperatures T_eff <= 475 K. The first
published observations provide important information: not only is the
atmospheric chemistry out of equilibrium, as previously known, but the
pressure-temperature profile is not in the standard adiabatic form. The rapid
rotation of these Jupiter-size, isolated, brown dwarfs dominates the
atmospheric dynamics, and thermal and compositional changes disrupt convection.
These processes produce a colder lower atmosphere, and a warmer upper
atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021)
presented empirical models where the pressure-temperature profile was adjusted
so that synthetic spectra reproduced the 1 <= lambda um <= 20 spectral energy
distributions of brown dwarfs with 260 <= T_eff K <= 540. We show that spectra
generated by these models fit the first JWST Y dwarf spectrum better than
standard-adiabat models. Unexpectedly, there is no 4.3 um PH_3 feature in the
JWST spectrum and atmospheres without phosphorus better reproduce the 4 um flux
peak. Our analysis of new JWST photometry indicates that the recently
discovered faint secondary of the WISE J033605.05-014350AB system
(Calissendorff et al. 2023) has T_eff = 295 K, making it the first dwarf in the
significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all
other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are
recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is
publicly available. Photometric color transformations are provided in an
Appendix.Comment: Accepted for publication in ApJ on 25 September 202
Atmospheric Analysis of the M/L- and M/T-Dwarf Binary Systems LHS 102 and Gliese 229
We present 0.9-2.5um spectroscopy with R~800 and 1.12-1.22um spectroscopy
with R~5800 for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS
102B. We also report IZJHKL' photometry for both components of the LHS 102
system, and L' photometry for Gl 229A. The data are combined with previously
published spectroscopy and photometry to produce flux distributions for each
component of the kinematically old disk M/L-dwarf binary system LHS 102 and the
kinematically young disk M/T-dwarf binary system Gliese 229. The data are
analyzed using synthetic spectra generated by the latest "AMES-dusty" and
"AMES-cond" models by Allard & Hauschildt. Although the models are not able to
reproduce the overall slope of the infrared flux distribution of the L dwarf,
most likely due to the treatment of dust in the photosphere, the data for the M
dwarfs and the T dwarf are well matched. We find that the Gl 229 system is
metal-poor despite having kinematics of the young disk, and that the LHS 102
system has solar metallicity. The observed luminosities and derived
temperatures and gravities are consistent with evolutionary model predictions
if the Gl 229 system is very young (age ~30 Myr) with masses (A,B) of
(0.38,>0.007)M(sun), and the LHS 102 system is older, aged 1-10 Gyr with masses
(A,B) of (0.19,0.07)M(sun).Comment: 29 pages incl. 13 figures and 5 tables;; accepted for publication in
MNRA
Analysis of a Very Massive DA White Dwarf via the Trigonometric Parallax and Spectroscopic Methods
By two different methods, we show that LHS 4033 is an extremely massive white
dwarf near its likely upper mass limit for destruction by unstable electron
captures. From the accurate trigonometric parallax reported herein, the
effective temperature (T=10,900 K) and the stellar radius (R=0.00368 R_sun) are
directly determined from the broad-band spectral energy distribution -- the
parallax method. The effective temperature and surface gravity are also
estimated independently from the simultaneous fitting of the observed Balmer
line profiles with those predicted from pure-hydrogen model atmospheres -- the
spectroscopic method (T=10,760 K, log g=9.46). The mass of LHS 4033 is then
inferred from theoretical mass-radius relations appropriate for white dwarfs.
The parallax method yields a mass estimate of 1.310--1.330 M_sun, for interior
compositions ranging from pure magnesium to pure carbon, respectively, while
the spectroscopic method yields an estimate of 1.318--1.335 M_sun for the same
core compositions. This star is the most massive white dwarf for which a robust
comparison of the two techniques has been made.Comment: 17 pages, including 4 figures, Accepted for Ap.
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