1,609 research outputs found
The IACOB project: I. Rotational velocities in Northern Galactic O and early B-type stars revisited. The impact of other sources of line-broadening
Stellar rotation is an important parameter in the evolution of massive stars.
Accurate and reliable measurements of projected rotational velocities in large
samples of OB stars are crucial to confront the predictions of stellar
evolutionary models with observational constraints. We reassess previous
determinations of projected rotational velocities (vsini) in Galactic OB stars
using a large, high quality spectroscopic dataset, and a strategy which account
for other sources of broadening appart from rotation affecting the diagnostic
lines We present a versatile and user friendly IDL tool, based on a combined
Fourier Transform (FT) + goodness of fit (GOF) methodology, for the
line-broadening characterization in OB-type stars. We use this tool to (a)
investigate the impact of macroturbulent and microturbulent broadenings on
vsini measurements, and (b) determine vsini in a sample of 200 Galactic OB-type
stars, also characterizing the amount of macroturbulent broadening (\vmacro)
affecting the line profiles. We present observational evidence illustrating the
strengths and limitations of the proposed FT+GOF methodology for the case of OB
stars. We confirm previous statements (based on indirect arguments or smaller
samples) that the macroturbulent broadening is ubiquitous in the massive star
domain. We compare the newly derived vsini with previous determinations not
accounting for this extra line-broadening contribution, and show that those
cases with vsini< 120 km/s need to be systematically revised downwards by ~25
(+/-20) km/s. We suggest that microturbulence may impose an upper limit below
which vsini and \vmacro\ could be incorrectly derived by means of the proposed
methodology as presently used, and discuss the implications of this statement
on the study of relatively narrow line massive stars.Comment: Accepted for publication in A&A (19 pages, 15 figures, 6 tables).
Tables A1-A5 will be make available in the final edited version of the paper
(or under request to SS-D
Numerical Distribution Functions for Seasonal Unit Root Tests
When working with time series data observed at intervals smaller than a year, it is often necessary to test for the presence of seasonal unit roots. One of the most widely used methods for testing seasonal unit roots is that of HEGY, which provides test statistics with non-standard distributions. This paper describes a generalisation of this method for any periodicity and uses a response surface regressions approach to calculate the critical values and P values of the HEGY statistics whatever the periodicity and sample size of the data. The algorithms are prepared with the Gretl open source econometrics package and some new tables of critical values for daily, hourly and half-hourly data are presented.seasonality, unit roots, surface response analysis
The IACOB spectroscopic database of Galactic OB stars
We present the IACOB spectroscopic database, the largest homogeneous database
of high-resolution, high signal-to-noise ratio spectra of Northern Galactic
OB-type stars compiled up to date. The spectra were obtained with the FIES
spectrograph attached to the Nordic Optical Telescope. We briefly summarize the
main characeristics and present status of the IACOB, first scientific results,
and some future plans for its extension and scientific exploitation.Comment: 2 pages. Poster contribution to the proceedings of the IAU272 "Active
OB stars: structure, evolution, mass loss and critical limits
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
Our aim is to find and classify OB stars in Sextans A, to later determine
accurate stellar parameters of these blue massive stars in this low metallicity
region .
Using UBV photometry, the reddening-free index Q and GALEX imaging, we built
a list of blue massive star candidates in Sextans A. We obtained low resolution
(R 1000) GTC-OSIRIS spectra for a fraction of them and carried out
spectral classification. For the confirmed O-stars we derive preliminary
stellar parameters.
The target selection criteria and observations were successful and have
produced the first spectroscopic atlas of OB-type stars in Sextans A. From the
whole sample of 18 observed stars, 12 were classified as early OB-types,
including 5 O-stars. The radial velocities of all target stars are in agreement
with their Sextans A membership, although three of them show significant
deviations. We determined the stellar parameters of the O-type stars using the
stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale.
Two of the O-stars are consistent with relatively strong winds and enhanced
helium abundances, although results are not conclusive. We discuss the position
of the OB stars in the HRD. Initial stellar masses run from slightly below 20
up to 40 solar masses.
The target selection method worked well for Sextans A, confirming the
procedure developed in Garcia \& Herrero (2013). The stellar temperatures are
consistent with findings in other galaxies. Some of the targets deserve
follow-up spectroscopy because of indications of a runaway nature, an enhanced
helium abundance or a relatively strong wind. We observe a correlation between
HI and OB associations similar to the irregular galaxy IC1613, confirming the
previous result that the most recent star formation of Sextans A is currently
on-going near the rim of the H\,{\sc I} cavity
Desarrollo urbano y estrategias de supervivencia en la periferia de la ciudad de México: Chalco, una aproximación antropológica.
Sin resume
Numerical Distribution Functions for Seasonal Unit Root Tests
When working with time series data observed at intervals smaller than a year, it is often necessary to test for the presence of seasonal unit roots. One of the most widely used methods for testing seasonal unit roots is that of HEGY, which provides test statistics with non-standard distributions. This paper describes a generalisation of this method for any periodicity and uses a response surface regressions approach to calculate the critical values and P values of the HEGY statistics whatever the periodicity and sample size of the data. The algorithms are prepared with the Gretl open source econometrics
package and some new tables of critical values for daily, hourly and half-hourly data are presented.Financial support from research project ECO2010-15332 from Ministerio de Ciencia e Innovación, and Econometrics Research Group IT-334-07 from the Basque Government are gratefully acknowledged. The SGI/IZO-SGIker UPV/EHU is gratefully aknowledged for its generous allocation of computational resource
Open cluster Dolidze 25: Stellar parameters and the metallicity in the Galactic Anticentre
The young open cluster Dolidze 25, in the direction of the Galactic
Anticentre, has been attributed a very low metallicity, with typical abundances
between and dex below solar. We intend to derive accurate cluster
parameters and accurate stellar abundances for some of its members. We have
obtained a large sample of intermediate- and high-resolution spectra for stars
in and around Dolidze 25. We used the FASTWIND code to generate stellar
atmosphere models to fit the observed spectra. We derive stellar parameters for
a large number of OB stars in the area, and abundances of oxygen and silicon
for a number of stars with spectral types around B0. We measure low abundances
in stars of Dolidze 25. For the three stars with spectral types around B0, we
find dex (Si) and dex (O) below the values typical in the solar
neighbourhood. These values, even though not as low as those given previously,
confirm Dolidze 25 and the surrounding H II region Sh2-284 as the most
metal-poor star-forming environment known in the Milky Way. We derive a
distance kpc to the cluster (kpc).
The cluster cannot be older than Myr, and likely is not much younger.
One star in its immediate vicinity, sharing the same distance, has Si and O
abundances at most dex below solar. The low abundances measured in
Dolidze 25 are compatible with currently accepted values for the slope of the
Galactic metallicity gradient, if we take into account that variations of at
least dex are observed at a given radius. The area traditionally
identified as Dolidze 25 is only a small part of a much larger star-forming
region that comprises the whole dust shell associated with Sh2-284 and very
likely several other smaller H II regions in its vicinity.Comment: Accepted for publication in Astronomy \& Astrophysics. 16 pages, 12
figure
Observational evidence for a correlation between macroturbulent broadening and line-profile variations in OB Supergiants
The spectra of O and B supergiants are known to be affected by a significant
form of extra line broadening (usually referred to as macroturbulence) in
addition to that produced by stellar rotation. Recent analyses of high
resolution spectra have shown that the interpretation of this line broadening
as a consequence of large scale turbulent motions would imply highly supersonic
velocity fields in photospheric regions, making this scenario quite improbable.
Stellar oscillations have been proposed as a likely alternative explanation. As
part of a long term observational project, we are investigating the
macroturbulent broadening in O and B supergiants and its possible connection
with spectroscopic variability phenomena and stellar oscillations. In this
letter, we present the first encouraging results of our project, namely firm
observational evidence for a strong correlation between the extra broadening
and photospheric line-profile variations in a sample of 13 supergiants with
spectral types ranging from O9.5 to B8.Comment: 8 pages, 3 figures, accepted for publication in ApJ
Spectroscopic and physical parameters of Galactic O-type stars. II. Observational constraints on projected rotational and extra broadening velocities as a function of fundamental parameters and stellar evolution
Rotation is of key importance for the evolution of hot massive stars,
however, the rotational velocities of these stars are difficult to determine.
Based on our own data for 31 Galactic O stars and incorporating similar data
for 86 OB supergiants from the literature, we aim at investigating the
properties of rotational and extra line-broadening as a function of stellar
parameters and at testing model predictions about the evolution of stellar
rotation. Fundamental stellar parameters were determined by means of the code
FASTWIND. Projected rotational and extra broadening velocities originate from a
combined Ft + GOF method. Model calculations published previously were used to
estimate the initial evolutionary masses. The sample O stars with Minit > 50
Msun rotate with less that 26% of their break-up velocity, and they also lack
objects with v sin i 35 Msun on the
hotter side of the bi-stability jump, the observed and predicted rotational
rates agree quite well; for those on the cooler side of the jump, the measured
velocities are systematically higher than the predicted ones. In general, the
derived extra broadening velocities decrease toward cooler Teff, whilst for
later evolutionary phases they appear, at the same v sin i, higher for
high-mass stars than for low-mass ones. None of the sample stars shows extra
broadening velocities higher than 110 km/s. For the majority of the more
massive stars, extra broadening either dominates or is in strong competition
with rotation. Conclusions: For OB stars of solar metallicity, extra broadening
is important and has to be accounted for in the analysis. When appearing at or
close to the zero-age main sequence, most of the single and more massive stars
rotate slower than previously thought. Model predictions for the evolution of
rotation in hot massive stars may need to be updated.Comment: 15 pages, 10 figures, accepted for publication in A &
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