49 research outputs found
The Dunhuang chinese sky: a comprehensive study of the oldest known star atlas
This paper presents an analysis of the star atlas included in the medieval
Chinese manuscript (Or.8210/S.3326), discovered in 1907 by the archaeologist
Aurel Stein at the Silk Road town of Dunhuang and now held in the British
Library. Although partially studied by a few Chinese scholars, it has never
been fully displayed and discussed in the Western world. This set of sky maps
(12 hour angle maps in quasi-cylindrical projection and a circumpolar map in
azimuthal projection), displaying the full sky visible from the Northern
hemisphere, is up to now the oldest complete preserved star atlas from any
civilisation. It is also the first known pictorial representation of the
quasi-totality of the Chinese constellations. This paper describes the history
of the physical object - a roll of thin paper drawn with ink. We analyse the
stellar content of each map (1339 stars, 257 asterisms) and the texts
associated with the maps. We establish the precision with which the maps are
drawn (1.5 to 4 degrees for the brightest stars) and examine the type of
projections used. We conclude that precise mathematical methods were used to
produce the atlas. We also discuss the dating of the manuscript and its
possible author and confirm the dates 649-684 (early Tang dynasty) as most
probable based on available evidence. This is at variance with a prior estimate
around +940. Finally we present a brief comparison with later sky maps, both in
China and in Europe.Comment: 19 pages, 5 Tables, 8 Figure
Spectroscopic monitoring of the Herbig Ae star HD 104237. II. Non-radial pulsations, mode analysis and fundamental stellar parameters
Herbig Ae/Be stars are intermediate-mass pre-main sequence (PMS) stars
showing signs of intense activity and strong stellar winds, whose origin is not
yet understood in the frame of current theoretical models of stellar evolution
for young stars. The evolutionary tracks of the earlier Herbig Ae stars cross a
recently discovered PMS instability strip. Many of these stars exhibit
pulsations of delta Scuti type. HD 104237 is a well-known pulsating Herbig Ae
star. In this article, we reinvestigated an extensive high-resolution
quasi-continuous spectroscopic data set in order to search for very faint
indications of non-radial pulsations in the line profile. To do this, we worked
on dynamical spectra of equivalent photospheric (LSD) profiles of HD 104237. A
2D Fourier analysis (F2D) was performed of the entire profile and the temporal
variation of the central depth of the line was studied with the time-series
analysis tools Period04 and SigSpec. We present a mode identification
corresponding to the detected dominant frequency. We perform a new accurate
determination of the fundamental stellar parameters in view of a forthcoming
asteroseismic modeling. Following the previous studies on this star, our
analysis of the dynamical spectrum of recentered LSD profiles corresponding to
the 22nd -25th of April 1999 nights spectra has confirmed the presence of
multiple oscillation modes of low-degree l in HD 104237 and led to the first
direct detection of a non-radial pulsation mode in this star: the dominant mode
F1 was identified by the Fourier 2D method having a degree l value comprised
between 1 and 2, the symmetry of the pattern variation indicating an azimuthal
order of +1 or -1. The detailed study of the fundamental stellar parameters has
provided a Teff, log g and iron abundance of 8550 +/- 150K, 3.9 +/- 0.3 and
-4.38 +/- 0.19 (i.e. [Fe/H]=+0.16 +/- 0.19), respectively
Fine structure of the chromospheric activity in Solar-type stars - The Halpha Line
A calibration of H-alpha as both a chromospheric diagnostic and an age
indicator is presented, complementing the works previously done on this subject
(Herbig 1985, Pasquini & Pallavicini 1991. The chromospheric diagnostic was
built with a statistically significant sample, covering nine years of
observations, and including 175 solar neighborhood stars. Regarding the age
indicator, the presence of stars for which very accurate ages are determined,
such as those belonging to clusters and kinematic groups, lends confidence to
our analysis. We also investigate the possibility that stars of the same age
might have gone through different tracks of chromospheric decay, identifying -
within the same age range - effects of metallicity and mass. These parameters,
however, as well as age, seem to be significant only for dwarf stars, losing
their meaning when we analyze stars in the subgiant branch. This result
suggests that, in these evolved stars, the emission mechanism cannot be
magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al.
2002c, and references therein). The Sun is found to be a typical star in its
H-alpha chromospheric flux, for its age, mass and metallicity. As a byproduct
of this work, we developed an automatic method to determine temperatures from
the wings of H-alpha, which means the suppression of the error inherent to the
visual procedure used in the literature.Comment: 10 pages, 10 figures, accepted for publication in Astronomy &
Astrophysics. Nature of replacement: match astro-ph and ADS title (greek
letter
Optical spectroscopic variability of Herbig Ae/Be stars
We analysed 337 multi-epoch optical spectra of 38 Herbig Ae/Be (HAeBe) stars
to gain insights into the variability behaviour of the circumstellar (CS)
atomic gas. Equivalent widths (EWs) and line fluxes of the Halpha, [OI]6300,
HeI5876 and NaID lines were obtained for each spectrum; the Halpha line width
at 10% of peak intensity (W10) and profile shapes were also measured and
classified. The mean line strengths and relative variabilities were quantified
for each star. Simultaneous optical photometry was used to estimate the line
fluxes.
We present a homogeneous spectroscopic database of HAeBe stars. The lines are
variable in practically all stars and timescales, although 30 % of the objects
show a constant EW in [OI]6300, which is also the only line that shows no
variability on timescales of hours. The HeI5876 and NaID EW relative
variabilities are typically the largest, followed by those in [OI]6300 and
Halpha. The EW changes can be larger than one order of magnitude for the
HeI5876 line, and up to a factor 4 for Halpha. The [OI]6300 and Halpha EW
relative variabilities are correlated for most stars in the sample. The Halpha
mean EW and W10 are uncorrelated, as are their relative variabilities. The
Halpha profile changes in 70 % of the objects. The massive stars in the sample
usually show more stable Halpha profiles with blueshifted self-absorptions and
less variable 10% widths.
Our data suggest multiple causes for the different line variations, but the
[OI]6300 and Halpha variability must share a similar origin in many objects.
The physical mechanism responsible for the Halpha line broadening does not
depend on the amount of emission; unlike in lower-mass stars, physical
properties based on the Halpha luminosity and W10 would significantly differ.
Our results provide additional support to previous works that reported
different physical mechanisms in Herbig Ae and Herbig Be stars.Comment: 10 pages, 5 figures, 2 appendixe
Ubiquitous outflows in DEEP2 spectra of star-forming galaxies at z=1.4
Galactic winds are a prime suspect for the metal enrichment of the
intergalactic medium and may have a strong influence on the chemical evolution
of galaxies and the nature of QSO absorption line systems. We use a sample of
1406 galaxy spectra at z~1.4 from the DEEP2 redshift survey to show that
blueshifted Mg II 2796, 2803 A absorption is ubiquitous in starforming galaxies
at this epoch. This is the first detection of frequent outflowing galactic
winds at z~1. The presence and depth of absorption are independent of AGN
spectral signatures or galaxy morphology; major mergers are not a prerequisite
for driving a galactic wind from massive galaxies. Outflows are found in
coadded spectra of galaxies spanning a range of 30x in stellar mass and 10x in
star formation rate (SFR), calibrated from K-band and from MIPS IR fluxes. The
outflows have column densities of order N_H ~ 10^20 cm^-2 and characteristic
velocities of ~ 300-500 km/sec, with absorption seen out to 1000 km/sec in the
most massive, highest SFR galaxies. The velocities suggest that the outflowing
gas can escape into the IGM and that massive galaxies can produce
cosmologically and chemically significant outflows. Both the Mg II equivalent
width and the outflow velocity are larger for galaxies of higher stellar mass
and SFR, with V_wind ~ SFR^0.3, similar to the scaling in low redshift
IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy
population at z~1 indicates that galactic winds occur in the progenitors of
massive spirals as well as those of ellipticals. The increase of outflow
velocity with mass and SFR constrains theoretical models of galaxy evolution
that include feedback from galactic winds, and may favor momentum-driven models
for the wind physics.Comment: Accepted by ApJ. 25 pages, 17 figures. Revised to add discussions of
intervening absorbers and AGN-driven outflows; conclusions unchange
The magnetic field of the pre-main sequence Herbig Ae star HD 190073
The general context of this paper is the study of magnetic fields in the
pre-main sequence intermediate mass Herbig Ae/Be stars. Magnetic fields are
likely to play an important role in pre-main sequence evolution at these
masses, in particular in controlling the gains and losses of stellar angular
momentum. The particular aim of this paper is to announce the detection of a
structured magnetic field in the Herbig Ae star HD 190073, and to discuss
various scenarii for the geometry of the star, its environment and its magnetic
field. We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to
obtain high-resolution and signal-to-noise circular polarization spectra which
demonstrate unambiguously the presence of a magnetic field in the photosphere
of this star. Nine circular polarization spectra were obtained, each one
showing a clear Zeeman signature. This signature is suggestive of a magnetic
field structured on large scales. The signature, which corresponds to a
longitudinal magnetic field of 74+- 10 G, does not vary detectably on a
one-year timeframe, indicating either an azimuthally symmetric field, a zero
inclination angle between the rotation axis and the line of sight, or a very
long rotation period. The optical spectrum of HD 190073 exhibits a large number
of emission lines. We discuss the formation of these emission lines in the
framework of a model involving a turbulent heated region at the base of the
stellar wind, possibly powered by magnetic accretion. This magnetic detection
brings an important element for our understanding of stellar magnetism at
intermediate masses.Comment: 10 pages, 6 figures. accepted by Astronomy & Astrophysic
CNO in evolved intermediate mass stars
In order to investigate the possible influence of rotation on the efficiency
of the first dredge-up we determined atmospheric parameters, masses, and
abundances of carbon, nitrogen, and oxygen in a sample of evolved intermediate
mass stars. We used high resolution spectra and conducted a model atmosphere
analysis. The abundances were calculated through spectral synthesis and
compared to the predictions of rotating and non-rotating evolutionary models.
Almost all those objects in our sample where carbon and nitrogen abundances
could be determined show signs of internal mixing. The stars, however, seem to
be mixed to different extents. Among the mixed stars we identify five in our
sample with abundances in agreement with the non-rotating models, four stars
that seem to be mixed beyond that, and one star that seems to be slightly less
mixed than predicted for the first dredge-up. There are also five stars that
seem to be slightly more mixed than expected, but their abundances are in
marginal agreement with both rotating and non-rotating models. Such differences
in the extent of the mixing are not predicted by the standard models and imply
the action of other mixing mechanisms than solely the convective dredge-up. We
also identified for the first time an important correlation between the [N/C]
ratio and the stellar mass.Comment: Accepted for publication in A&A. Final version with language
correction
High Resolution X-ray Spectroscopy of T Tauri Stars in the Taurus-Auriga Complex
Differences have been reported between the X-ray emission of accreting and non-accreting stars. Some observations have suggested that accretion shocks could be responsible for part of the X-ray emission in Classical T Tauri stars (CTTS). We present high-resolution X-ray spectroscopy of nine pre-main sequence stars in order to test the proposed spectroscopic differences between accreting and non-accreting pre-main sequence stars. We use X-ray spectroscopy from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. We interpret the spectra using optically thin thermal models with variable abundances, together with an absorption column density. For BP Tau and AB Aur we derive electron densities from the O VII triplets. Using the O VII/O VIII count ratios as a diagnostic for cool plasma, we find that CTTS display a soft excess (with equivalent electron temperatures of ~ 2.5-3 MK) when compared with WTTS or zero-age main-sequence stars. Although the O VII triplet in BP Tau is consistent with a high electron density (3.4 x 10^11 cm^-3), we find a low density for the accreting Herbig star AB Aur (n_e < 10^10 cm^-3). The element abundances of accreting and non-accreting stars are similar. The Ne abundance is found to be high (4-6 times the Fe abundance) in all K and M-type stars. In contrast, for the three G-type stars (SU Aur, HD 283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4-0.8 times solar photospheric values) compared to later-type stars. Adding the results from our sample to former high-resolution studies of T Tauri stars, we find a soft excess in all accreting stars, but in none of the non-accretors. On the other hand, high electron density and high Ne/Fe abundance ratios do not seem to be present in all accreting pre-main sequence stars
The Spectral Energy Distribution and Mass-loss Rate of the A-Type Supergiant Deneb
A stellar wind module has been developed for the PHOENIX stellar atmosphere
code for the purpose of computing non-LTE, line-blanketed, expanding
atmospheric structures and detailed synthetic spectra of hot luminous stars
with winds. We apply the code to observations of Deneb, for which we report the
first positive detections of mm and cm emission (obtained using the SCUBA and
the VLA), as well a strong upper limit on the 850 micron flux (using the HHT).
The slope of the radio spectrum shows that the stellar wind is partially
ionized. We report a uniform-disk angular diameter measurement, 2.40 +/- 0.06
mas, from the Navy Prototype Optical Interferometer (NPOI). The measured
bolometric flux and corrected NPOI angular diameter yield an effective
temperature of 8600 +/- 500 K. Least-squares comparisons of synthetic spectral
energy distributions from 1220 A to 3.6 cm with the observations provide
estimates for the effective temperature and the mass-loss rate of 8400 +/- 100
K and 8 +/- 3 E-7 M_sun/yr, respectively. This range of mass-loss rates is
consistent with that derived from high dispersion UV spectra when non-LTE
metal-line blanketing is considered. We are unable achieve a reasonable fit to
a typical Halpha P-Cygni profile with any model parameters over a reasonable
range. This is troubling because the \ha profile is the observational basis for
Wind Momentum-Luminosity Relationship.Comment: Accepted by the Astrophysical Journal, 43 pages, 23 figure
The first high-resolution X-ray spectrum of a Herbig Star: The case of AB Aurigae
We present the first high-resolution X-ray spectrum of a prototypical Herbig
star (AB Aurigae), measure and interpret various spectral features, and compare
our results with model predictions. We use X-ray spectroscopy data from
XMM-Newton. The spectra are interpreted using thermal, optically thin emission
models with variable element abundances and a photoelectric absorption
component. We interpret line flux ratios in He-like triplet of O VII as a
function of electron density and the UV radiation field. We use the nearby
co-eval classical T Tauri star SU Aur as a comparison. AB Aurigae reveals a
soft X-ray spectrum, most plasma being concentrated at 1-6 MK. The He-like
triplet reveals no signatures of increased densities and there are no clear
indications for strong abundance anomalies. The light curve displays modulated
variability, with a period of ~ 42 hr. It is unlikely that a nearby, undetected
lower-mass companion is the source of the X-rays. Accretion shocks close to the
star should be irradiated by the photosphere, leading to alteration in the
He-like triplet fluxes of O VII, which we do not measure. Also, no indications
for high densities are found, although the mass accretion rate is presently
unknown. Emission from wind shocks is unlikely, given the weak radiation
pressure. A possible explanation would be a solar-like magnetic corona.
Magnetically confined winds provide a very promising alternative. The X-ray
period is indeed close to periods previously measured in optical lines from the
wind.Comment: 18 pages, 7 Figure