537 research outputs found
Fundamental Properties of O-Type Stars
We present a comprehensive analysis of high-resolution, far-UV HST/STIS,
FUSE, and optical spectra of 17 O stars in the SMC. Our analysis is based on
NLTE metal line-blanketed model atmospheres calculated with our NLTE code
TLUSTY. We systematically explore the sensitivity of various UV and optical
lines to different stellar parameters. We have obtained consistent fits of the
UV and the optical spectrum to derive the effective temperature, surface
gravity, surface composition, and microturbulent velocity of each star. Stellar
radii, masses, luminosities and ages then follow. Similarly to more limited
recent studies, we derive cooler temperatures than the standard Teff
calibration of O stars. We propose a new calibration between the spectral type
and effective temperature based on our results from UV metal lines as well as
optical hydrogen and helium lines. For stars of the same spectral subtype, we
find a general good agreement between Teff determinations obtained with TLUSTY,
CMFGEN, and FASTWIND models. We derive ionizing luminosities that are smaller
by a factor of 3 compared to luminosities inferred from previous standard
calibrations. The chemical composition analysis reveals that the surface of
about 3/4 of the program stars is moderately to strongly enriched in nitrogen,
while showing the original helium, carbon, and oxygen abundances. Our results
support the new stellar evolution models that predict that the surface of fast
rotating stars becomes N-rich during the main sequence phase because of
rotationally-induced mixing. Most stars exhibit the ``mass discrepancy''
problem. This discrepancy too is a result of fast rotation which lowers the
measured effective gravity. Our study thus emphasizes the importance of
rotation in our understanding of the properties of massive stars. (abridged)Comment: Submitted to Astrophysical Journal; 69 page
An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 um) and the coronagraph on the
Near-InfraRed Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space
Telescope (HST) to survey 45 single stars with a median age of 0.15 Gyr, an
average distance of 30 pc, and an average H-magnitude of 7 mag. For the median
age we were capable of detecting a 30 M_Jup companion at separations between 15
and 200 AU. A 5 M_Jup object could have been detected at 30 AU around 36% of
our primaries. For several of our targets that were less than 30 Myr old, the
lower mass limit was as low as a Jupiter mass, well into the high mass planet
region. Results of the entire survey include the proper motion verification of
five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B) and one
possible brown dwarf binary (Gl 577B/C).Comment: 11 figures, accepted by A
Searching for WR stars in I Zw 18 -- The origin of HeII emission
I Zw 18 is the most metal poor star-forming galaxy known and is an ideal
laboratory to probe stellar evolution theory at low metallicities. Using
archival HST WFPC2 imaging and FOS spectroscopy we were able to improve
previous studies. We constructed a continuum free HeII map, which was used to
identify Wolf-Rayet (WR) stars recently found by ground-based spectroscopy and
to locate diffuse nebular emission. Most of the HeII emission is associated
with the NW stellar cluster, clearly displaced from the surrounding shell-like
[OIII] and Halpha emission. We found evidence for HeII sources, compatible with
5--9 WNL stars and/or compact nebular HeII emission, as well as residual
diffuse emission. Only one of them is outside the NW cluster. We have
calculated evolutionary tracks for massive stars and synthesis models at the
appropriate metallicity (Z ~ 0.02 Zsun). These single star models predict a
mass limit M_WR ~ 90 Msun for WR stars to become WN and WC/WO. For an
instantaneous burst model with a Salpeter IMF extending up to M_up ~ 120-150
Msun our model predictions are in reasonable agreement with the observed
equivalent widths. Our model is also able to fully reproduce the observed
equivalent widths of nebular HeII emission due to the presence of WC/WO stars.
This quantitative agreement and the spatial correlation of nebular HeII with
the stellar cluster and the position of WR stars shown from the ground-based
spectra further supports the hypothesis that WR stars are responsible for
nebular HeII emission in extra-galactic HII regions. (Abridged abstract)Comment: Accepted by ApJ. LaTeX using aas2pp4, psfigs macros. 23 pages
including 6 figures. Paper and figures also separately available at
http://www.obs-mip.fr/omp/astro/people/schaerer
The Low-Redshift Lyman Alpha Forest toward PKS 0405-123
We present STIS 7km/s resolution data of the Ly-a forest toward PKS0405-123
(z=0.574). We make two samples. The strong sample (60 systems) has column
density logNHI>13.3 over 0.002<z<0.423, and the weak sample (44) has
logNHI>13.1 at 0.020<z<0.234; 7 absorbers show metals, all with OVI, but often
offset in velocity from HI. The strong sample Doppler parameters show
=47+-22km/s; for the weak, =44+-21km/s. Line blending and s/n effects
likely inflate b. The redshift density dN/dz is consistent with previous, lower
resolution results for logNHI>14.0. For 13.1<logNHI<14.0, we find a dN/dz
overdensity of ~0.2-0.3 dex at 0.127<z<0.234, which we believe arises from
cosmic variance. We find Ly-a clustering over Delta v13.3,
consistent with a numerical model. There is a void in the strong sample at
0.0320<z<0.0814. We detect radial velocity correlations for Delta v<250km/s
between Ly-a absorbers and 39 galaxies in a 10' field. The correlation grows
with minimum NHI, peaking for logNHI ~> 13.5-14.0. The strength is similar to
that of the galaxy-galaxy correlation for our sample, implying that such Ly-a
systems have mass log M/Msun= 11.3+1.0-0.6. Including lower column density
systems in the sample shows correlations only out to Delta v <125km/s, as would
be expected for smaller density perturbations. Galaxy counts and local HI
column density appear correlated, peaking at ~5000km/s. We also give column
densities for Galactic species. (Abridged)Comment: 59 pages, 6 tables, 14 figures, 1 standalone, ApJ, in press; minor
revisions/clarifications for acceptance, including galaxy lis
Social preferences, accountability, and wage bargaining
We assess the extent of preferences for employment in a collective wage bargaining situation with heterogeneous workers. We vary the size of the union and introduce a treatment mechanism transforming the voting game into an individual allocation task. Our results show that highly productive workers do not take employment of low productive workers into account when making wage proposals, regardless of whether insiders determine the wage or all workers. The level of pro-social preferences is small in the voting game, while it increases as the game is transformed into an individual allocation task. We interpret this as an accountability effect
Complex C: A Low-Metallicity High-Velocity Cloud Plunging into the Milky Way
(Abridged) We present a new high-resolution (7 km/s FWHM) echelle spectrum of
3C 351 obtained with STIS. 3C 351 lies behind the low-latitude edge of
high-velocity cloud Complex C, and the new spectrum provides accurate
measurements of O I, Si II, Al II, Fe II, and Si III absorption lines at the
velocity of the HVC. We use collisional and photoionization models to derive
ionization corrections; in both models we find that the overall metallicity Z =
0.1 - 0.3 Z_{solar} in Complex C, but nitrogen must be underabundant. The iron
abundance indicates that Complex C contains very little dust. The absorbing gas
probably is not gravitationally confined. The gas could be pressure-confined by
an external medium, but alternatively we may be viewing the leading edge of the
HVC, which is ablating and dissipating as it plunges into the Milky Way. O VI
column densities observed with FUSE toward nine QSOs/AGNs behind Complex C
support this conclusion: N(O VI) is highest near 3C 351, and the O VI/H I ratio
increases substantially with decreasing latitude, suggesting that the
lower-latitude portion of the cloud is interacting more vigorously with the
Galaxy. The other sight lines through Complex C show some dispersion in
metallicity, but with the current uncertainties, the measurements are
consistent with a constant metallicity throughout the HVC. However, all of the
Complex C sight lines require significant nitrogen underabundances. Finally, we
compare the 3C 351 sight line to the sight line to the nearby QSO H1821+643 to
search for evidence of outflowing Galactic fountain gas that could be mixing
with Complex C. We find that the intermediate-velocity gas detected toward 3C
351 and H1821+643 has a higher metallicity and may well be a fountain/chimney
outflow from the Perseus spiral arm.Comment: Submitted to AJ. Figures 1-4 compressed for astro-ph; better quality
figures are available at
http://astro.princeton.edu/~tripp/astro/qualitypreps/complexc.ps.g
The VLT-FLAMES survey of massive stars: Mass loss and rotation of early-type stars in the SMC
We have studied the optical spectra of a sample of 31 O- and early B-type
stars in the Small Magellanic Cloud, 21 of which are associated with the young
massive cluster NGC 346. Stellar parameters are determined using an automated
fitting method. Comparison with predictions of stellar evolution that account
for stellar rotation does not result in a unique age, though most stars are
best represented by an age of 1-3 Myr. The present day v_sini distribution of
the 21 dwarf stars in our sample is consistent with an underlying rotational
velocity (v_r) distribution that can be characterised by a mean velocity of
about 160-190 km/s and an effective half width of 100-150 km/s. The v_r
distribution must include a small percentage of slowly rotating stars. If
predictions of the time evolution of the equatorial velocity for massive stars
within the environment of the SMC are correct, the young age of the cluster
implies that this underlying distribution is representative for the initial
rotational velocity distribution. The location in the Hertzsprung-Russell
diagram of the stars showing helium enrichment is in qualitative agreement with
evolutionary tracks accounting for rotation, but not for those ignoring v_r.
The mass loss rates of the SMC objects having luminosities of log L/L_sun > 5.4
are in excellent agreement with predictions. However, for lower luminosity
stars the winds are too weak to determine M_dot accurately from the optical
spectrum. Two of three spectroscopically classified Vz stars from our sample
are located close to the theoretical zero age main sequence, as expected.Comment: 35 pages, accepted for publication in A&
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