2,055 research outputs found
Abundances of metal-weak thick-disc candidates
High resolution spectra of 5 candidate metal-weak thick-disc stars suggested
by Beers & Sommer-Larsen (1995) are analyzed to determine their chemical
abundances. The low abundance of all the objects has been confirmed with
metallicity reaching [Fe/H]=-2.9. However, for three objects, the astrometric
data from the Hipparcos catalogue suggests they are true halo members. The
remaining two, for which proper-motion data are not available, may have
disc-like kinematics. It is therefore clear that it is useful to address
properties of putative metal-weak thick-disc stars only if they possess full
kinematic data. For CS 22894-19 the abundance pattern similar to those of
typical halo stars is found, suggesting that chemical composition is not a
useful discriminant between thick-disc and halo stars. CS 29529-12 is found to
be C enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s
process elements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr]
considerably larger than what is found in more metal-rich carbon-rich stars,
but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a).
Hipparcos data have been used to calculate the space velocities of 25 candidate
metal-weak thick-disc stars, thus allowing us to identify 3 bona fide members,
which support the existence of a metal-poor tail of the thick-disc, at variance
with a claim to the contrary by Ryan & Lambert (1995).Comment: to be published in MNRA
First Attempt at Spectroscopic Detection of Gravity Modes in a Long-Period Pulsating Subdwarf B Star -- PG 1627+017
In the first spectroscopic campaign for a PG 1716 variable (or long-period
pulsating subdwarf B star), we succeeded in detecting velocity variations due
to g-mode pulsations at a level of 1.0-1.5 km/s.The observations were obtained
during 40 nights on 2-m class telescopes in Arizona, South Africa,and
Australia. The target,PG1627+017, is one of the brightest and largest amplitude
stars in its class.It is also the visible component of a post-common envelope
binary.Our final radial velocity data set includes 84 hours of time-series
spectroscopy over a time baseline of 53 days. Our derived radial velocity
amplitude spectrum, after subtracting the orbital motion, shows three potential
pulsational modes 3-4 sigma above the mean noise level, at 7201.0s,7014.6s and
7037.3s.Only one of the features is statistically likely to be real,but all
three are tantalizingly close to, or a one day alias of, the three strongest
periodicities found in the concurrent photometric campaign. We further
attempted to detect pulsational variations in the Balmer line amplitudes. The
single detected periodicity of 7209 s, although weak, is consistent with
theoretical expectations as a function of wavelength.Furthermore, it allows us
to rule out a degree index of l= 3 or l= 5 for that mode. Given the extreme
weakness of g-mode pulsations in these stars,we conclude that anything beyond
simply detecting their presence will require larger telescopes,higher
efficiency spectral monitoring over longer time baselines,improved longitude
coverage, and increased radial velocity precision.Comment: 39 pages, 9 figures, 4 tables, ApJ accepted. See postscript for full
abtrac
Improved determination of the atmospheric parameters of the pulsating sdB star Feige 48
As part of a multifaceted effort to exploit better the asteroseismological
potential of the pulsating sdB star Feige 48, we present an improved
spectroscopic analysis of that star based on new grids of NLTE, fully
line-blanketed model atmospheres. To that end, we gathered four high S/N
time-averaged optical spectra of varying spectral resolution from 1.0 \AA\ to
8.7 \AA, and we made use of the results of four independent studies to fix the
abundances of the most important metals in the atmosphere of Feige 48. The mean
atmospheric parameters we obtained from our four spectra of Feige 48 are :
Teff= 29,850 60 K, log = 5.46 0.01, and log N(He)/N(H) =
2.88 0.02. We also modeled for the first time the He II line at 1640
\AA\ from the STIS archive spectrum of the star and we found with this line an
effective temperature and a surface gravity that match well the values obtained
with the optical data. With some fine tuning of the abundances of the metals
visible in the optical domain we were able to achieve a very good agreement
between our best available spectrum and our best-fitting synthetic one. Our
derived atmospheric parameters for Feige 48 are in rather good agreement with
previous estimates based on less sophisticated models. This underlines the
relatively small effects of the NLTE approach combined with line blanketing in
the atmosphere of this particular star, implying that the current estimates of
the atmospheric parameters of Feige 48 are reliable and secure.Comment: Accepted for publication in ApJ, April 201
The orbits of subdwarf-B + main-sequence binaries. II. Three eccentric systems; BD+29 3070, BD +34 1543 and Feige 87
The predicted orbital-period distribution of the subdwarf-B (sdB) population
is bi-modal with a peak at short ( 250 days) periods.
Observationally, many short-period sdB systems are known, but the predicted
long period peak is missing as orbits have only been determined for a few
long-period systems. As these predictions are based on poorly understood
binary-interaction processes, it is of prime importance to confront the
predictions with reliable observational data. We therefore initiated a
monitoring program to find and characterize long-period sdB stars. In this
paper we aim to determine the orbital parameters of the three long-period
sdB+MS binaries BD+29 3070, BD+34 1543 and Feige 87, to constrain their
absolute dimensions and the physical parameters of the components.
High-resolution spectroscopic time series were obtained with HERMES at the
Mercator telescope on La Palma, and analyzed to determine the radial velocities
of both the sdB and MS components. Photometry from the literature was used to
construct the spectral-energy distribution (SED) of the binaries. Atmosphere
models were used to fit these SEDs and to determine the surface gravities and
temperatures of both components of all systems. Spectral analysis was used to
check the results of the SEDs. An orbital period of 1283 +- 63 d, a mass ratio
of q = 0.39 +- 0.04 and a significant non-zero eccentricity of e = 0.15 +- 0.01
were found for BD+29 3070. For BD+34 1543 we determined P = 972 +- 2 d, q =
0.57 +- 0.01 and again a clear non-zero eccentricity of e = 0.16 +- 0.01. Last,
for Feige 87 we found P = 936 +- 2 d, q = 0.55 +- 0.01 and e = 0.11 +- 0.01.
BD+29 3070, BD+34 1543 and Feige 87 are long period sdB + MS binaries on
clearly eccentric orbits. These results are in conflict with the predictions of
stable Roche-lobe overflow models.Comment: 15 pages, 6 figures, Accepted by A&
Radiative levitation: a likely explanation for pulsations in the unique hot O subdwarf star SDSS J160043.6+074802.9
Context. SDSS J160043.6+074802.9 (J1600+0748 for short) is the only hot sdO star for which unambiguous multiperiodic luminosity variations have been reported so far. These rapid variations, with periods in the range from ~60 s to ~120 s, are best qualitatively explained in terms of pulsational instabilities, but the exact nature of the driving mechanism has remained a puzzle.
Aims. Our primary goal is to examine quantitatively how pulsation modes can be excited in an object such as J1600+0748. Given the failure of uniform-metallicity models as well documented in the recent Ph.D. thesis of C. RodrĂguez-LĂłpez, we consider the effects of radiative levitation on iron as a means to boost the efficiency of the opacity-driving mechanism in models of J1600+0748.
Methods. We combine high sensitivity time-averaged optical spectroscopy and full nonadiabatic calculations to carry out our study. In the first instance, this is used to estimate the location of J1600+0748 in the log plane. Given this essential input, we pulsate stellar models consistent with these atmospheric parameters. We construct both uniform-metallicity models and structures in which the iron abundance is specified by the condition of diffusive equilibrium between gravitational settling and radiative levitation.
Results. On the basis of NTLE H/He synthetic spectra, we find that the target star has the following atmospheric parameters: log g = 5.93 0.11, = 71 070 2725 K, and log N(He)/N(H) = -0.85 0.08. This takes into account our deconvolution of the spectrum of J1600+0748 as it is polluted by the light of a main sequence companion. We confirm that uniform-metallicity stellar models with Z in the range from 0.02 to 0.10 cannot excite pulsation modes of the kind observed. On the other hand, we find that the inclusion of radiative levitation, as we implemented it, leads to pulsational instabilities in a period range that overlaps with, although it is narrower than, the observed range in J1600+0748. The excited modes correspond to low-order, low-degree p-modes.
Conclusions. We infer that radiative levitation is a likely essential ingredient in the excitation physics at work in J1600+0748
The Multiplicity of Ahonhom (Spirits) in the Akan Spiritual Cosmology
African tradition and thought consider spirits to be elements of power, force, authority, and vital energy underlying all existence. There is therefore this huge fear of what goes on in the unseen world since spirits are known to operate in this realm. Invisible though this power may be, Africans perceive it directly. This has led to the adoption of all manner of strategies in order to get closer to OnyankopÉn, âthe Supreme Beingâ so that the necessary support and protection may be received. Without the spirits, the Akan feels that there is no way to reach God. In this paper I will discuss the role of OnyankopÉn, âthe Supreme Beingâ and some other spirit beings in the Akan spiritual cosmology and the various roles they play in the affairs of human. Key words: OnyankopÉn, ahonhom, Akan, ancestors, deities, spirits, aboso
High-resolution spectropolarimetric observations of hot subdwarfs
We report on high-resolution spectropolarimetric observations of the hot
subdwarf stars HD 76431 and Feige 66, using the ESPaDOnS echelle
spectropolarimeter at CFHT. We compute cross-correlation Stokes I and V line
profiles to enhance the signal-to-noise ratio. We then average all available
cross-correlation profiles of each star to further decrease the noise level.
Although both targets were previously reported to host kilo-gauss magnetic
fields, we do not derive any evidence of large-scale photospheric fields from
our sets of observations, in spite of tight error bars on the longitudinal
field of the order of 60 gauss for HD 76431 and 200 gauss for Feige 66. A new
analysis of FORS1 observations of HD 76431, which provided the basis for the
original claim of field detection, confirms the absence of any detectable
Zeeman signature, with an error bar of about 100 gauss on the longitudinal
magnetic field.Comment: Proceedings of the fifth meeting on hot subdwarf stars and related
object
Discovery of a second pulsating intermediate helium-enriched sdOB star
We present the discovery of long-period, low-amplitude, g-mode pulsations in
the intermediate He-rich hot subdwarf (sdOB) star Feige 46. Up until now only
one other He-enriched sdOB star (LS IV -14116) was known to exhibit such
pulsations. From our ground-based light curves of Feige 46, we extracted five
independent periodicities ranging from 2294 s to 3400 s. We fitted our
low-resolution, high signal-to-noise optical spectrum of the star with our grid
of non-LTE model atmospheres and derived the following atmospheric parameters:
Teff = 36120 230 K, log g = 5.93 0.04 and log N(He)/N(H) = -0.32
0.03 (formal fitting errors only). These parameters are very similar to
those of LS IV -14116 and place Feige 46 well outside of the instability strip
where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia
parallax and proper motion of Feige 46 to perform a kinematic analysis of this
star and found that it likely belongs to the Galactic halo population. This is
most certainly an intriguing and interesting result given that LS IV -14116 is
also a halo object. The mechanism responsible for the pulsations in these two
peculiar objects remains unclear but a possible scenario involves the
-mechanism. Although they are the only two members in their class of
variable stars, these pulsators appear to have more in common than just their
pulsation properties.Comment: Accepted for publication in A&
- âŠ