44,341 research outputs found
The Payne: self-consistent ab initio fitting of stellar spectra
We present The Payne, a general method for the precise and simultaneous
determination of numerous stellar labels from observed spectra, based on
fitting physical spectral models. The Payne combines a number of important
methodological aspects: it exploits the information from much of the available
spectral range; it fits all labels (stellar parameters and element abundances)
simultaneously; it uses spectral models, where the atmosphere structure and the
radiative transport are consistently calculated to reflect the stellar labels.
At its core The Payne has an approach to accurate and precise interpolation and
prediction of the spectrum in high-dimensional label-space, which is flexible
and robust, yet based on only a moderate number of ab initio models (O(1000)
for 25 labels). With a simple neural-net-like functional form and a suitable
choice of training labels, this interpolation yields a spectral flux prediction
good to rms across a wide range of and log g (including
dwarfs and giants). We illustrate the power of this approach by applying it to
the APOGEE DR14 data set, drawing on Kurucz models with recently improved line
lists: without recalibration, we obtain physically sensible stellar parameters
as well as 15 element abundances that appear to be more precise than the
published APOGEE DR14 values. In short, The Payne is an approach that for the
first time combines all these key ingredients, necessary for progress towards
optimal modelling of survey spectra; and it leads to both precise and accurate
estimates of stellar labels, based on physical models and without
re-calibration. Both the codes and catalog are made publicly available online.Comment: 22 pages, 17 figures, 2 tables, ApJ (Accepted for publication- 2019
May 11
The NIRSPEC Ultracool Dwarf Radial Velocity Survey
We report the results of an infrared Doppler survey designed to detect brown
dwarf and giant planetary companions to a magnitude-limited sample of ultracool
dwarfs. Using the NIRSPEC spectrograph on the Keck II telescope, we obtained
approximately 600 radial velocity measurements over a period of six years for a
sample of 59 late-M and L dwarfs spanning spectral types M8/L0 to L6. A
subsample of 46 of our targets have been observed on three or more epochs. We
rely on telluric CH4 absorption features in the Earth's atmosphere as a
simultaneous wavelength reference and exploit the rich set of CO absorption
features found in the K-band spectra of cool stars and brown dwarfs to measure
radial velocities and projected rotational velocities. For a bright, slowly
rotating M dwarf standard we demonstrate a radial velocity precision of 50 m/s,
and for slowly rotating L dwarfs we achieve a typical radial velocity precision
of approximately 200 m/s. This precision is sufficient for the detection of
close-in giant planetary companions to mid-L dwarfs as well as more equal mass
spectroscopic binary systems with small separations (a<2 AU). We present an
orbital solution for the subdwarf binary LSR1610-0040 as well as an improved
solution for the M/T binary 2M0320-04. We also combine our radial velocity
measurements with distance estimates and proper motions from the literature to
estimate the dispersion of the space velocities of the objects in our sample.
Using a kinematic age estimate we conclude that our UCDs have an age of
5.0+0.7-0.6 Gyr, similar to that of nearby sun-like stars. We simulate the
efficiency with which we detect spectroscopic binaries and find that the rate
of tight (a<1 AU) binaries in our sample is 2.5+8.6-1.6%, consistent with
recent estimates in the literature of a tight binary fraction of 3-4%.
(abridged)Comment: 39 pages, 20 figures. Accepted for publication in Ap
Galaxy properties from J-PAS narrow-band photometry
We study the consistency of the physical properties of galaxies retrieved
from SED-fitting as a function of spectral resolution and signal-to-noise ratio
(SNR). Using a selection of physically motivated star formation histories, we
set up a control sample of mock galaxy spectra representing observations of the
local universe in high-resolution spectroscopy, and in 56 narrow-band and 5
broad-band photometry. We fit the SEDs at these spectral resolutions and
compute their corresponding the stellar mass, the mass- and luminosity-weighted
age and metallicity, and the dust extinction. We study the biases,
correlations, and degeneracies affecting the retrieved parameters and explore
the r\^ole of the spectral resolution and the SNR in regulating these
degeneracies. We find that narrow-band photometry and spectroscopy yield
similar trends in the physical properties derived, the former being
considerably more precise. Using a galaxy sample from the SDSS, we compare more
realistically the results obtained from high-resolution and narrow-band SEDs
(synthesized from the same SDSS spectra) following the same spectral fitting
procedures. We use results from the literature as a benchmark to our
spectroscopic estimates and show that the prior PDFs, commonly adopted in
parametric methods, may introduce biases not accounted for in a Bayesian
framework. We conclude that narrow-band photometry yields the same trend in the
age-metallicity relation in the literature, provided it is affected by the same
biases as spectroscopy; albeit the precision achieved with the latter is
generally twice as large as with the narrow-band, at SNR values typical of the
different kinds of data.Comment: 26 pages, 15 figures. Accepted for publication in MNRA
PASTIS: Bayesian extrasolar planet validation II. Constraining exoplanet blend scenarios using spectroscopic diagnoses
The statistical validation of transiting exoplanets proved to be an efficient
technique to secure the nature of small exoplanet signals which cannot be
established by purely spectroscopic means. However, the spectroscopic diagnoses
are providing us with useful constraints on the presence of blended stellar
contaminants. In this paper, we present how a contaminating star affects the
measurements of the various spectroscopic diagnoses as function of the
parameters of the target and contaminating stars using the model implemented
into the PASTIS planet-validation software. We find particular cases for which
a blend might produce a large radial velocity signal but no bisector variation.
It might also produce a bisector variation anti-correlated with the radial
velocity one, as in the case of stellar spots. In those cases, the full width
half maximum variation provides complementary constraints. These results can be
used to constrain blend scenarios for transiting planet candidates or radial
velocity planets. We review all the spectroscopic diagnoses reported in the
literature so far, especially the ones to monitor the line asymmetry. We
estimate their uncertainty and compare their sensitivity to blends. Based on
that, we recommend the use of BiGauss which is the most sensitive diagnosis to
monitor line-profile asymmetry. In this paper, we also investigate the
sensitivity of the radial velocities to constrain blend scenarios and develop a
formalism to estimate the level of dilution of a blended signal. Finally, we
apply our blend model to re-analyse the spectroscopic diagnoses of HD16702, an
unresolved face-on binary which exhibits bisector variations.Comment: Accepted for publication in MNRA
A precise asteroseismic age and radius for the evolved Sun-like star KIC 11026764
The primary science goal of the Kepler Mission is to provide a census of
exoplanets in the solar neighborhood, including the identification and
characterization of habitable Earth-like planets. The asteroseismic
capabilities of the mission are being used to determine precise radii and ages
for the target stars from their solar-like oscillations. Chaplin et al. (2010)
published observations of three bright G-type stars, which were monitored
during the first 33.5 days of science operations. One of these stars, the
subgiant KIC 11026764, exhibits a characteristic pattern of oscillation
frequencies suggesting that it has evolved significantly. We have derived
asteroseismic estimates of the properties of KIC 11026764 from Kepler
photometry combined with ground-based spectroscopic data. We present the
results of detailed modeling for this star, employing a variety of independent
codes and analyses that attempt to match the asteroseismic and spectroscopic
constraints simultaneously. We determine both the radius and the age of KIC
11026764 with a precision near 1%, and an accuracy near 2% for the radius and
15% for the age. Continued observations of this star promise to reveal
additional oscillation frequencies that will further improve the determination
of its fundamental properties.Comment: 16 pages, 6 figures, 4 tables, ApJ in pres
3D-HST: A wide-field grism spectroscopic survey with the Hubble Space Telescope
We present 3D-HST, a near-infrared spectroscopic Treasury program with the
Hubble Space Telescope for studying the processes that shape galaxies in the
distant Universe. 3D-HST provides rest-frame optical spectra for a sample of
~7000 galaxies at 1<z<3.5, the epoch when 60% of all star formation took place,
the number density of quasars peaked, the first galaxies stopped forming stars,
and the structural regularity that we see in galaxies today must have emerged.
3D-HST will cover 3/4 (625 sq.arcmin) of the CANDELS survey area with two
orbits of primary WFC3/G141 grism coverage and two to four parallel orbits with
the ACS/G800L grism. In the IR these exposure times yield a continuum
signal-to-noise of ~5 per resolution element at H~23.1 and a 5sigma emission
line sensitivity of 5x10-17 erg/s/cm2 for typical objects, improving by a
factor of ~2 for compact sources in images with low sky background levels. The
WFC3/G141 spectra provide continuous wavelength coverage from 1.1-1.6 um at a
spatial resolution of ~0."13, which, combined with their depth, makes them a
unique resource for studying galaxy evolution. We present the preliminary
reduction and analysis of the grism observations, including emission line and
redshift measurements from combined fits to the extracted grism spectra and
photometry from ancillary multi-wavelength catalogs. The present analysis
yields redshift estimates with a precision of sigma(z)=0.0034(1+z), or
sigma(v)~1000 km/s. We illustrate how the generalized nature of the survey
yields near-infrared spectra of remarkable quality for many different types of
objects, including a quasar at z=4.7, quiescent galaxies at z~2, and the most
distant T-type brown dwarf star known. The CANDELS and 3D-HST surveys combined
will provide the definitive imaging and spectroscopic dataset for studies of
the 1<z<3.5 Universe until the launch of the James Webb Space Telescope.Comment: Replacement reflects version now accepted by ApJS. A preliminary data
release intended to provide a general illustration of the WFC3 grism data is
available at http://3dhst.research.yale.edu
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