3,588 research outputs found
On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars
The Fourier Transform method is a popular tool to derive the rotational
velocities of stars from their spectral line profiles. However, its domain of
validity does not include line-profile variables with time-dependent profiles.
We investigate the performance of the method for such cases, by interpreting
the line-profile variations of spotted B stars, and of pulsating B tars, as if
their spectral lines were caused by uniform surface rotation along with
macroturbulence. We perform time-series analysis and harmonic least-squares
fitting of various line diagnostics and of the outcome of several
implementations of the Fourier Transform method. We find that the projected
rotational velocities derived from the Fourier Transform vary appreciably
during the pulsation cycle whenever the pulsational and rotational velocity
fields are of similar magnitude. The macroturbulent velocities derived while
ignoring the pulsations can vary with tens of km/s during the pulsation cycle.
The temporal behaviour of the deduced rotational and macroturbulent velocities
are in antiphase with each other. The rotational velocity is in phase with the
second moment of the line profiles. The application of the Fourier method to
stars with considerable pulsational line broadening may lead to an appreciable
spread in the values of the rotation velocity, and, by implication, of the
deduced value of the macroturbulence. These two quantities should therefore not
be derived from single snapshot spectra if the aim is to use them as a solid
diagnostic for the evaluation of stellar evolution models of slow to moderate
rotators.Comment: 13 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
The rotation rates of massive stars: How slow are the slow ones?
Context: Rotation plays a key role in the life cycles of stars with masses
above 8 Msun. Hence, accurate knowledge of the rotation rates of such massive
stars is critical for understanding their properties and for constraining
models of their evolution. Aims: This paper investigates the reliability of
current methods used to derive projected rotation speeds v sin i from
line-broadening signatures in the photospheric spectra of massive stars,
focusing on stars that are not rapidly rotating. Methods: We use slowly
rotating magnetic O-stars with well-determined rotation periods to test the
Fourier transform (FT) and goodness-of-fit (GOF) methods typically used to
infer projected rotation rates of massive stars. Results: For our two magnetic
test stars with measured rotation periods longer than one year, i.e., with v
sin i < 1 km/s, we derive v sin i ~ 40-50 km/s from both the FT and GOF
methods. These severe overestimates are most likely caused by an insufficient
treatment of the competing broadening mechanisms referred to as microturbulence
and macroturbulence. Conclusions: These findings warn us not to rely
uncritically on results from current standard techniques to derive projected
rotation speeds of massive stars in the presence of significant additional line
broadening, at least when v sin i <~ 50 km/s. This may, for example, be crucial
for i) determining the statistical distribution of observed rotation rates of
massive stars, ii) interpreting the evolutionary status and spin-down histories
of rotationally braked B-supergiants, and iii) explaining the deficiency of
observed O-stars with spectroscopically inferred v sin i ~ 0 km/s. Further
investigations of potential shortcomings of the above techniques are presently
under way.Comment: 4 pages, 4 figures, accepted for publication in A&A Letter
No temperature fluctuations in the giant HII region H 1013
While collisionally excited lines in HII regions allow one to easily probe
the chemical composition of the interstellar medium in galaxies, the possible
presence of important temperature fluctuations casts some doubt on the derived
abundances. To provide new insights into this question, we have carried out a
detailed study of a giant HII region, H 1013, located in the galaxy M101, for
which many observational data exist and which has been claimed to harbour
temperature fluctuations at a level of t^2 = 0.03-0.06. We have first
complemented the already available optical observational datasets with a
mid-infrared spectrum obtained with the Spitzer Space Telescope. Combined with
optical data, this spectrum provides unprecedented information on the
temperature structure of this giant HII region. A preliminary analysis based on
empirical temperature diagnostics suggests that temperature fluctuations should
be quite weak. We have then performed a detailed modelling using the pyCloudy
package based on the photoionization code Cloudy. We have been able to produce
photoionization models constrained by the observed Hb surface brightness
distribution and by the known properties of the ionizing stellar population
than can account for most of the line ratios within their uncertainties. Since
the observational constraints are both strong and numerous, this argues against
the presence of significant temperature fluctuations in H 1013. The oxygen
abundance of our best model is 12 + log O/H = 8.57, as opposed to the values of
8.73 and 8.93 advocated by Esteban et al. (2009) and Bresolin (2007),
respectively, based on the significant temperature fluctuations they derived.
However, our model is not able to reproduce the intensities of the oxygen
recombination lines . This cannot be attributed to observational uncertainties
and requires an explanation other than temperature fluctuations.Comment: accepted in Astronomy & Astrophysic
The massive multiple system HD 64315
The O6 Vn star HD 64315 is believed to belong to the star-forming region
known as NGC 2467, but previous distance estimates do not support this
association. We explore the multiple nature of this star with the aim of
determining its distance, and understanding its connection to NGC 2467. A total
of 52 high-resolution spectra have been gathered over a decade. We use their
analysis, in combination with the photometric data from All Sky Automated
Survey and Hipparcos catalogues, to conclude that HD 64315 is composed of at
least two spectroscopic binaries, one of which is an eclipsing binary. HD 64315
contains two binary systems, one of which is an eclipsing binary. The two
binaries are separated by 0.09 arcsec (or 500 AU) if the most likely distance
to the system, around 5 kpc, is considered. The presence of fainter companions
is not excluded by current observations. The non-eclipsing binary (HD 64315
AaAb) has a period of 2.70962901+/-0.00000021 d. Its components are hotter than
those of the eclipsing binary, and dominate the appearance of the system. The
eclipsing binary (HD 64315 BaBb) has a shorter period of 1.0189569+/-0.0000008
d. We derive masses of 14.6+-2.3 M for both components of the BaBb
system. They are almost identical; both stars are overfilling their respective
Roche lobes, and share a common envelope in an overcontact configuration. The
non-eclipsing binary is a detached system composed of two stars with spectral
types around O6 V with minimum masses of 10.8 M and 10.2 M, and
likely masses aprox. 30 M. HD 64315 provides a cautionary tale about
high-mass star isolation and multiplicity. Its total mass is likely above 90
M,but it seems to have formed without an accompanying cluster. It
contains one the most massive overcontact binaries known, a likely merger
progenitor in a very wide multiple system.Comment: 14 pages, 13 figures, 8 Table
Surface abundances of ON stars
Massive stars burn hydrogen through the CNO cycle during most of their
evolution. When mixing is efficient, or when mass transfer in binary systems
happens, chemically processed material is observed at the surface of O and B
stars. ON stars show stronger lines of nitrogen than morphologically normal
counterparts. Whether this corresponds to the presence of material processed
through the CNO cycle or not is not known. Our goal is to answer this question.
We perform a spectroscopic analysis of a sample of ON stars with atmosphere
models. We determine the fundamental parameters as well as the He, C, N, and O
surface abundances. We also measure the projected rotational velocities. We
compare the properties of the ON stars to those of normal O stars. We show that
ON stars are usually helium-rich. Their CNO surface abundances are fully
consistent with predictions of nucleosynthesis. ON stars are more chemically
evolved and rotate - on average - faster than normal O stars. Evolutionary
models including rotation cannot account for the extreme enrichment observed
among ON main sequence stars. Some ON stars are members of binary systems, but
others are single stars as indicated by stable radial velocities. Hence, mass
transfer is not a simple explanation for the observed chemical properties. We
conclude that ON stars show extreme chemical enrichment at their surface,
consistent with nucleosynthesis through the CNO cycle. Its origin is not clear
at present.Comment: 18 pages, 10 figures (+ appendix). A&A accepte
Extreme mass ratios and fast rotation in three massive binaries
The origin of rapid rotation in massive stars remains debated, although
binary interactions are now often advocated as a cause. However, the broad and
shallow lines in the spectra of fast rotators make direct detection of binarity
difficult. In this paper, we report on the discovery and analysis of
multiplicity for three fast-rotating massive stars: HD25631 (B3V), HD191495
(B0V), and HD46485 (O7V). They display strikingly similar TESS light curves,
with two narrow eclipses superimposed on a sinusoidal variation due to
reflection effects. We complement these photometric data by spectroscopy from
various instruments (X-Shooter, Espadons, FUSE...), to further constrain the
nature of these systems. The detailed analyses of these data demonstrates that
the companions of the massive OB stars have low masses (~1Msol) with rather
large radii (2-4 Rsol) and low temperatures (<15 kK). These companions display
no UV signature, which would exclude a hot subdwarf nature, but disentangling
of the large set of X-Shooter spectra of HD25631 revealed the typical signature
of chromospheric activity in the companion's spectrum. In addition, despite the
short orbital periods (P=3-7d), the fast-rotating OB-stars still display
non-synchronized rotation and all systems appear young (<20Myr). This suggests
that, as in a few other cases, these massive stars are paired in those systems
with non-degenerate, low-mass PMS companions, implying that fast rotation would
not be a consequence of a past binary interactions in their case.Comment: accepted for publication by MNRA
Evidence of magnetic field decay in massive main-sequence stars
A significant fraction of massive main-sequence stars show strong,
large-scale magnetic fields. The origin of these fields, their lifetimes, and
their role in shaping the characteristics and evolution of massive stars are
currently not well understood. We compile a catalogue of 389 massive
main-sequence stars, 61 of which are magnetic, and derive their fundamental
parameters and ages. The two samples contain stars brighter than magnitude 9 in
the V band and range in mass between 5 and 100 Msun. We find that the
fractional main-sequence age distribution of all considered stars follows what
is expected for a magnitude limited sample, while that of magnetic stars shows
a clear decrease towards the end of the main sequence. This dearth of old
magnetic stars is independent of the choice of adopted stellar evolution
tracks, and appears to become more prominent when considering only the most
massive stars. We show that the decreasing trend in the distribution is
significantly stronger than expected from magnetic flux conservation. We also
find that binary rejuvenation and magnetic suppression of core convection are
unlikely to be responsible for the observed lack of older magnetic massive
stars, and conclude that its most probable cause is the decay of the magnetic
field, over a time span longer than the stellar lifetime for the lowest
considered masses, and shorter for the highest masses. We then investigate the
spin-down ages of the slowly rotating magnetic massive stars and find them to
exceed the stellar ages by far in many cases. The high fraction of very slowly
rotating magnetic stars thus provides an independent argument for a decay of
the magnetic fields.Comment: Accepted for publication on A&A; 9 pages, 8 figure
Spectral classification and properties of the O Vz stars in the Galactic O-Star Spectroscopic Survey (GOSSS)
On the basis of the Galactic O-Star Spectroscopic Survey (GOSSS), a detailed
systematic investigation of the O Vz stars is presented. The currently used
spectral classification criteria are rediscussed, and the Vz phenomenon is
recalibrated through the addition of a quantitative criterion based on the
equivalent widths of the He I 4471, He II 4542, and He II 4686 spectral lines.
The GOSSS O Vz and O V populations resulting from the newly adopted spectral
classification criteria are comparatively analyzed. The locations of the O Vz
stars are probed, showing a concentration of the most extreme cases toward the
youngest star forming regions. The occurrence of the Vz spectral peculiarity in
a solar-metallicity environment, as predicted by the fastwind code, is also
investigated, confirming the importance of taking into account several
processes for the correct interpretation of the phenomenon.Comment: Accepted for publication in The Astronomical Journa
A map of OMC-1 in CO 9-8
The distribution of 12C16O J=9-8 (1.037 THz) emission has been mapped in
OMC-1 at 35 points with 84" resolution. This is the first map of this source in
this transition and only the second velocity-resolved ground-based observation
of a line in the terahertz frequency band. There is emission present at all
points in the map, a region roughly 4' by 6' in size, with peak antenna
temperature dropping only near the edges. Away from the Orion KL outflow, the
velocity structure suggests that most of the emission comes from the OMC-1
photon-dominated region, with a typical linewidthof 3-6 km/s. Large velocity
gradient modeling of the emission in J=9-8 and six lower transitions suggests
that the lines originate in regions with temperatures around 120 K and
densities of at least 10^(3.5) cm^(-3) near theta^(1) C Ori and at the Orion
Bar, and from 70 K gas at around 10^(4) cm^(-3) southeast and west of the bar.
These observations are among the first made with the 0.8 m Smithsonian
Astrophysical Observatory Receiver Lab Telescope, a new instrument designed to
observe at frequencies above 1 THz from an extremely high and dry site in
northern Chile.Comment: Minor changes to references, text to match ApJ versio
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