130 research outputs found
Constraining the structure and formation of the Galactic bulge from a field in its outskirts. FLAMES-GIRAFFE spectra of about 400 red giants around (l,b)=(0{\deg},-10{\deg})
The presence of two stellar populations in the Milky Way bulge has been
reported recently. We aim at studying the abundances and kinematics of stars in
the outer bulge, thereby providing additional constraints on models of its
formation. Spectra of 401 red giant stars in a field at
(l,b)=(0{\deg},-10{\deg}) were obtained with FLAMES at the VLT. Stars of
luminosities down to below the two bulge red clumps (RCs) are included. From
these spectra we measure general metallicities, abundances of Fe and the
alpha-elements, and radial velocities (RV) of the stars. These measurements as
well as photometric data are compared to simulations with the Besancon and
TRILEGAL models of the Galaxy. We confirm the presence of two populations among
our sample stars: i) a metal-rich one at [M/H] ~+0.3, comprising about 30% of
the sample, with low RV dispersion and low alpha-abundance, and ii) a
metal-poor population at [M/H] ~-0.6 with high RV dispersion and high
alpha-abundance. The metal-rich population could be connected to the Galactic
bar. We identify this population as the carrier of the double RC feature. We do
not find a significant difference in metallicity or RV between the two RCs, a
small difference in metallicity being probably due to a selection effect. The
RV dispersion agrees well with predictions of the Besancon Galaxy model, but
the metallicity of the "thick bulge" model component should be shifted to lower
metallicity by 0.2 to 0.3dex to well reproduce the observations. We present
evidence that the metallicity distribution function depends on the evolutionary
state of the sample stars, suggesting that enhanced mass loss preferentially
removes metal-rich stars. We also confirm the decrease of \alpha-element
over-abundance with increasing metallicity.Comment: 19 pages (excluding on-line table), 21 figures, accepted for
publication in A&
Seismic inference of 57 stars using full-length Kepler data sets
We present stellar properties (mass, age, radius, distances) of 57 stars from
a seismic inference using full-length data sets from Kepler. These stars
comprise active stars, planet-hosts, solar-analogs, and binary systems. We
validate the distances derived from the astrometric Gaia-Tycho solution.
Ensemble analysis of the stellar properties reveals a trend of mixing-length
parameter with the surface gravity and effective temperature. We derive a
linear relationship with the seismic quantity to
estimate the stellar age. Finally, we define the stellar regimes where the
Kjeldsen et al (2008) empirical surface correction for 1D model frequencies is
valid.Comment: 4-page proceedings from Seismology of the Sun and the Distant Stars
2016, TASC/KASC, Azores, Portugal, corrected references in v
Radii, masses, and ages of 18 bright stars using interferometry and new estimations of exoplanetary parameters
Accurate stellar parameters are needed in numerous domains of astrophysics.
The position of stars on the H-R diagram is an important indication of their
structure and evolution, and it helps improve stellar models. Furthermore, the
age and mass of stars hosting planets are required elements for studying
exoplanetary systems. We aim at determining accurate parameters of a set of 18
bright exoplanet host and potential host stars from interferometric
measurements, photometry, and stellar models. Using the VEGA/CHARA
interferometer, we measured the angular diameters of 18 stars, ten of which
host exoplanets. We combined them with their distances to estimate their radii.
We used photometry to derive their bolometric flux and, then, their effective
temperature and luminosity to place them on the H-R diagram. We then used the
PARSEC models to derive their best fit ages and masses, with error bars derived
from MC calculations. Our interferometric measurements lead to an average of
1.9% uncertainty on angular diameters and 3% on stellar radii. There is good
agreement between measured and indirect estimations of angular diameters (from
SED fitting or SB relations) for MS stars, but not as good for more evolved
stars. For each star, we provide a likelihood map in the mass-age plane;
typically, two distinct sets of solutions appear (an old and a young age). The
errors on the ages and masses that we provide account for the metallicity
uncertainties, which are often neglected by other works. From measurements of
its radius and density, we also provide the mass of 55 Cnc independently of
models. From the stellar masses, we provide new estimates of semi-major axes
and minimum masses of exoplanets with reliable uncertainties. We also derive
the radius, density, and mass of 55 Cnc e, a super-Earth that transits its
stellar host. Our exoplanetary parameters reflect the known population of
exoplanets.Comment: 23 pages, 9 figures, published in A&A. (This version includes proof
corrections.
Detection of the Milky Way spiral arms in dust from 3D mapping
Large stellar surveys are sensitive to interstellar dust through the effects
of reddening. Using extinctions measured from photometry and spectroscopy,
together with three-dimensional (3D) positions of individual stars, it is
possible to construct a three-dimensional dust map. We present the first
continuous map of the dust distribution in the Galactic disk out to 7 kpc
within 100 pc of the Galactic midplane, using red clump and giant stars from
SDSS APOGEE DR14. We use a non-parametric method based on Gaussian Processes to
map the dust density, which is the local property of the ISM rather than an
integrated quantity. This method models the dust correlation between points in
3D space and can capture arbitrary variations, unconstrained by a pre-specified
functional form. This produces a continuous map without line-of-sight
artefacts. Our resulting map traces some features of the local Galactic spiral
arms, even though the model contains no prior suggestion of spiral arms, nor
any underlying model for the Galactic structure. This is the first time that
such evident arm structures have been captured by a dust density map in the
Milky Way. Our resulting map also traces some of the known giant molecular
clouds in the Galaxy and puts some constraints on their distances, some of
which were hitherto relatively uncertain.Comment: Accepted for publication in A&A, 9 pages, 7 figure
Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes
To study the demographics of interstellar ices in the Central Molecular Zone
(CMZ) of the Milky Way, we obtain near-infrared spectra of red point
sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and
mid-infrared photometry, including objects in the previous paper of this
series, to ensure that these sources trace a large amount of absorption through
clouds in each line of sight. We find that most of the sample ( objects)
show CO band-head absorption at m, tagging them as red (super-)
giants. Despite the photospheric signature, however, a fraction of the sample
with -band spectra () exhibit large HO ice column densities
(), and six of them also reveal CHOH ice
absorption. As one of such objects is identified as a young stellar object
(YSO) in our previous work, these ice-rich sight lines are likely associated
with background stars in projection to an extended envelope of a YSO or a dense
cloud core. The low frequency of such objects in the early stage of stellar
evolution implies a low star-formation rate ( yr),
reinforcing the previous claim on the suppressed star-formation activity in the
CMZ. Our data also indicate that the strong "shoulder" CO ice absorption at
m observed in YSO candidates in the previous paper arises from
CHOH-rich ice grains having a large CO concentration [].Comment: 28 pages, 12 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Massive Young Stellar Objects in the Galactic Center. I. Spectroscopic Identification from Spitzer/IRS Observations
We present results from our spectroscopic study, using the Infrared
Spectrograph (IRS) onboard the Spitzer Space Telescope, designed to identify
massive young stellar objects (YSOs) in the Galactic Center (GC). Our sample of
107 YSO candidates was selected based on IRAC colors from the high spatial
resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone
(CMZ), which spans the central ~300 pc region of the Milky Way Galaxy. We
obtained IRS spectra over 5um to 35um using both high- and low-resolution IRS
modules. We spectroscopically identify massive YSOs by the presence of a 15.4um
shoulder on the absorption profile of 15um CO2 ice, suggestive of CO2 ice mixed
with CH3OH ice on grains. This 15.4um shoulder is clearly observed in 16
sources and possibly observed in an additional 19 sources. We show that 9
massive YSOs also reveal molecular gas-phase absorption from CO2, C2H2, and/or
HCN, which traces warm and dense gas in YSOs. Our results provide the first
spectroscopic census of the massive YSO population in the GC. We fit YSO models
to the observed spectral energy distributions and find YSO masses of 8 - 23
Msun, which generally agree with the masses derived from observed radio
continuum emission. We find that about 50% of photometrically identified YSOs
are confirmed with our spectroscopic study. This implies a preliminary star
formation rate of ~0.07 Msun/yr at the GC.Comment: Accepted for publication in Ap
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