834 research outputs found
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
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 &
Bright OB stars in the Galaxy.II. Wind variability in O supergiants as traced by H-alpha
We investigate the line-profile variability (lpv) of H-alpha for a large
sample of O-type supergiants. We used the Temporal Variance Spectrum (TVS)
analysis, developed by Fullerton et al 1996 and modified by us to take into
account the effects of wind emission. By means of a comparative analysis we put
a number of constraints on the properties of the variability as a function of
stellar and wind parameters. The results of our analysis show that all the
stars in the sample show evidence of significant lpv in H-alpha, mostly
dominated by processes in the wind. The variations occur between zero and 0.3
v_inf (i.e., below ~1.5 R_star), in good agreement with the results from
similar studies. A comparison between observations and line-profile simulations
indicates that for stars with intermediate wind densities the H-alpha
variability can be explained by simple models, consisting of coherent or broken
shells (blobs) uniformly distributed over the wind volume, with an intrinsic
scatter in the maximum density contrast of about a factor of two. For stars at
lower and higher wind densities, on the other hand, we found certain
inconsistencies between observations and our predictions, most importantly
concerning the mean amplitude and the symmetry properties of the TVS. This
disagreement might be explained with the presence of coherent large-scale
structures, partly confined in a volume close to the star. Interpreted in terms
of a variable mass-loss rate, the observed variations of H-alpha indicate
changes of 4% with respect to the mean value of M_dot for stars with stronger
winds and of 16% for stars with weaker winds. The effect of these variations on
the corresponding wind momenta is rather insignificant (<0.16 dex), increasing
the local scatter without affecting the Wind Momentum Luminosity Relationship.Comment: 22 pages, 11 figures. Accepted by Astronomy and Astrophysic
Rotational properties of the O-type star population in the Tarantula region
The 30 Doradus (30\,Dor) region in the Large Magellanic Cloud (also known as
the Tarantula Nebula) is the nearest massive starburst region, containing the
richest sample of massive stars in the Local Group. It is the best possible
laboratory to investigate aspects of the formation and evolution of massive
stars. Here, we focus on rotation which is a key parameter in the evolution of
these objects. We establish the projected rotational velocity, ,
distribution of an unprecedented sample of 216 radial velocity constant
() O-type stars in 30\,Dor observed in
the framework of the VLT-FLAMES Tarantula Survey (VFTS). The distribution of
shows a two-component structure: a peak around 80 and a high-velocity tail extending up to 600 .
Around 75% of the sample has 0 200
with the other 25% distributed in the high-velocity tail. The presence of the
low-velocity peak is consistent with that found in other studies of late-O and
early-B stars. The high-velocity tail is compatible with expectations from
binary interaction synthesis models and may be predominantly populated by
post-binary interaction, spun-up, objects and mergers. This may have important
implications for the nature of progenitors of long-duration gamma ray bursts.Comment: 4 pages, 1 figure. Conference proceedings article: Massive stars:
from alpha to Omega, 10-14 June 2013, Rhodes, Greec
Rotational velocities of single and binary O-type stars in the Tarantula Nebula
Rotation is a key parameter in the evolution of massive stars, affecting
their evolution, chemical yields, ionizing photon budget, and final fate. We
determined the projected rotational velocity, , of 330 O-type
objects, i.e. 210 spectroscopic single stars and 110 primaries in
binary systems, in the Tarantula nebula or 30 Doradus (30\,Dor) region. The
observations were taken using VLT/FLAMES and constitute the largest homogeneous
dataset of multi-epoch spectroscopy of O-type stars currently available. The
most distinctive feature of the distributions of the
presumed-single stars and primaries in 30 Dor is a low-velocity peak at around
100\,. Stellar winds are not expected to have spun-down the
bulk of the stars significantly since their arrival on the main sequence and
therefore the peak in the single star sample is likely to represent the outcome
of the formation process. Whereas the spin distribution of presumed-single
stars shows a well developed tail of stars rotating more rapidly than
300\,, the sample of primaries does not feature such a
high-velocity tail. The tail of the presumed-single star distribution is
attributed for the most part -- and could potentially be completely due -- to
spun-up binary products that appear as single stars or that have merged. This
would be consistent with the lack of such post-interaction products in the
binary sample, that is expected to be dominated by pre-interaction systems. The
peak in this distribution is broader and is shifted toward somewhat higher spin
rates compared to the distribution of presumed-single stars. Systems displaying
large radial velocity variations, typical for short period systems, appear
mostly responsible for these differences.Comment: 6 pages, 3 figures, Proceedings IAU Symposium No. 307, 2014, 'New
windows on massive stars: asteroseismology, interferometry, and
spectropolarimetry
Wind structure of late B supergiants I. Multi-line analyses of near-surface and wind structure in HD 199 478 (B8 Iae)
We provide a quantitative analysis of time-variable phenomena in the
photospheric, near-star, and outflow regions of the late-B supergiant (SG) HD
199478. The analysis is based primarily on optical spectroscopic datasets
secured between 1999 and 2000 from the Bulgarian NAO, Tartu, and Ritter
Observatories. The temporal behaviour of HD 199478 is characterised by three
key empirical properties: (i) systematic central velocity shifts in the
photospheric absorption lines, including C II and He I, over a characteristic
time-scale of abou 20 days; (ii) extremely strong, variable H alpha emission
with no clear modulation signal, and (iii) the occurrence in 2000 of a (rare)
high-velocity absorption (HVA) event in H alpha, which evolved over about 60
days, showing the clear signature of mass infall and outflows. In these
properties HD 199478 resembles few other late-B SGs with peculiar emission and
HVAs in H alpha (HD 91619, HD 34085, HD 96919). Non-LTE line synthesis
modelling is conducted using FASTWIND for these late-B SGs to constrain and
compare their fundamental parameters within the context of extreme behaviour in
the H alpha lines. Our analysis indicate that at the cooler temperature edge of
B SGs, there are objects whose wind properties, as traced by H alpha, are
inconsistent with the predictions of the smooth, spherically symmetric wind
approximation. This discordance is still not fully understood and may highlight
the role of a non-spherical, disk-like, geometry, which may result from
magnetically-driven equatorial compression of the gas. Ordered dipole magnetic
fields may also lead to confined plasma held above the stellar surface, which
ultimately gives rise to transient HVA events.Comment: 12 pages. To be published in Astronomy and AStrophysic
The VLT-FLAMES Tarantula Survey XIX. B-type Supergiants - Atmospheric parameters and nitrogen abundances to investigate the role of binarity and the width of the main sequence
TLUSTY non-LTE model atmosphere calculations have been used to determine
atmospheric parameters and nitrogen (N) abundances for 34 single and 18 binary
B-type supergiants (BSGs). The effects of flux contribution from an unseen
secondary were considered for the binary sample. We present the first
systematic study of the incidence of binarity for a sample of BSGs across the
theoretical terminal age main sequence (TAMS). To account for the distribution
of effective temperatures of the BSGs it may be necessary to extend the TAMS to
lower temperatures. This is consistent with the derived distribution of mass
discrepancies, projected rotational velocities (vsini) and N abundances,
provided that stars cooler than this temperature are post RSG objects. For the
BSGs in the Tarantula and previous FLAMES surveys, most have small vsini. About
10% have larger vsini (>100 km/s) but surprisingly these show little or no N
enhancement. All the cooler BSGs have low vsini of <70km/s and high N abundance
estimates, implying that either bi-stability braking or evolution on a blue
loop may be important. A lack of cool binaries, possibly reflects the small
sample size. Single star evolutionary models, which include rotation, can
account for the N enhancement in both the single and binary samples. The
detailed distribution of N abundances in the single and binary samples may be
different, possibly reflecting differences in their evolutionary history. The
first comparative study of single and binary BSGs has revealed that the main
sequence may be significantly wider than previously assumed, extending to
Teff=20000K. Some marginal differences in single and binary atmospheric
parameters and abundances have been identified, possibly implying non-standard
evolution for some of the sample. This sample as a whole has implications for
several aspects of our understanding of the evolution of BSGs. Full abstract in
paperComment: 21 pages, 15 figures, 11 table
The VLT-FLAMES Tarantula Survey XVIII. Classifications and radial velocities of the B-type stars
We present spectral classifications for 438 B-type stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud. Radial velocities are provided for 307 apparently single stars, and for 99 targets with radial-velocity variations which are consistent with them being spectroscopic binaries. We investigate the spatial distribution of the radial velocities across the 30 Dor region, and use the results to identify candidate runaway stars. Excluding potential runaways and members of two older clusters in the survey region (SL 639 and Hodge 301), we determine a systemic velocity for 30 Dor of 271.6 ± 12.2 kms-1 from 273 presumed single stars. Employing a 3σ criterion we identify nine candidate runaway stars (2.9% of the single stars with radial-velocity estimates). The projected rotational velocities of the candidate runaways appear to be significantly different to those of the full B-type sample, with a strong preference for either large (≥345 kms-1) or small (≤65 kms-1) rotational velocities. Of the candidate runaways, VFTS 358 (classified B0.5: V) has the largest differential radial velocity (−106.9 ± 16.2 kms-1), and a preliminary atmospheric analysis finds a significantly enriched nitrogen abundance of 12 + log (N/H) ≳ 8.5. Combined with a large rotational velocity (ve sin i = 345 ± 22 kms-1), this is suggestive of past binary interaction for this star
The VLT-FLAMES Tarantula Survey XXXII. Low-luminosity late O-type stars -- classification, main physical parameters, and silicon abundances
Analysis of late O-type stars observed in the Large Magellanic Cloud (LMC) by
the VLT-FLAMES Tarantula Survey (VFTS) revealed a discrepancy between the
physical properties estimated from model-atmosphere analysis and those expected
from their morphological classifications. Here we revisit the analysis of 32 of
these puzzling objects using new hydrogen-helium-silicon FASTWIND models and a
different fitting approach to re-evaluate their physical properties. Our new
analysis confirms that these stars indeed have properties that are typical of
late O-type dwarfs. We also present the first estimates of silicon abundances
for O-type stars in the 30 Dor clusters NGC 2060 and NGC 2070, with a weighted
mean abundance for our sample of 7.05 +/- 0.03. Our values are about 0.20 dex
lower than those previously derived for B-type stars in the LMC clusters N 11
and NGC 2004 using TLUSTY models. Various possibilities (e.g. differences in
the analysis methods, effects of microturbulence, and real differences between
stars in different clusters) were considered to account for these results. We
also used our grid of FASTWIND models to reassess the impact of using the
Galactic classification criteria for late O-type stars in the LMC by
scrutinising their sensitivity to different stellar properties. At the cool
edge of the O star regime the HeII 4686/HeI 4713 ratio used to assign
luminosity class for Galactic stars can mimic giants or bright giants in the
LMC, even for objects with high gravities (log_g > 4.0 dex). We argue that this
line ratio is not a reliable luminosity diagnostic for late O-type stars in the
LMC, and that the SiIV 4989/HeI4026 ratio is more robust for these types.Comment: 18 pages, 10 figures accepted for publication in A&
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