Searcharxiv⌕ Search

arXiv subjects

N. Markova

Publications and source records attributed to N. Markova.

At least 37 records · Page 2Linked to original sources
arXiv2012

The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136

Detailed studies of resolved young massive star clusters are necessary to determine their dynamical state and evaluate the importance of gas expulsion and early cluster evolution. In an effort to gain insight into the dynamical state of the young massive cluster R136 and obtain the first measurement of its velocity dispersion, we analyse multi-epoch spectroscopic data of the inner regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the variability, we use the radial velocities of non-variable sources to place an upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within a projected distance of 5 pc from the centre of the cluster. After accounting for the contributions of undetected binaries and measurement errors through Monte Carlo simulations, we conclude that the true velocity dispersion is likely between 4 and 5 km/s given a range of standard assumptions about the binary distribution. This result is consistent with what is expected if the cluster is in virial equilibrium, suggesting that gas expulsion has not altered its dynamics. We find that the velocity dispersion would be ~25 km/s if binaries were not identified and rejected, confirming the importance of the multi-epoch strategy and the risk of interpreting velocity dispersion measurements of unresolved extragalactic young massive clusters.

astro-ph.GA↗
arXiv2012

The VLT-FLAMES Tarantula Survey VI: Evidence for rotation of the young massive cluster R136

Although it has important ramifications for both the formation of star clusters and their subsequent dynamical evolution, rotation remains a largely unexplored characteristic of young star clusters (few Myr). Using multi-epoch spectroscopic data of the inner regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of the VLT-FLAMES Tarantula Survey, we search for rotation of the young massive cluster R136. From the radial velocities of 36 apparently single O-type stars within a projected radius of 10 pc from the centre of the cluster, we find evidence, at the 95% confidence level, for rotation of the cluster as a whole. We use a maximum likelihood method to fit simple rotation curves to our data and find a typical rotational velocity of ~3 km/s. When compared to the low velocity dispersion of R136, our result suggests that star clusters may form with at least ~20% of the kinetic energy in rotation.

astro-ph.GA↗
arXiv2011

The VLT-FLAMES Tarantula Survey: The fastest rotating O-type star and shortest period LMC pulsar - remnants of a supernova disrupted binary?

We present a spectroscopic analysis of an extremely rapidly rotating late O-type star, VFTS102, observed during a spectroscopic survey of 30 Doradus. VFTS102 has a projected rotational velocity larger than 500\kms\ and probably as large as 600\kms; as such it would appear to be the most rapidly rotating massive star currently identified. Its radial velocity differs by 40\kms\ from the mean for 30 Doradus, suggesting that it is a runaway. VFTS102 lies 12 pcs from the X-ray pulsar PSR J0537-6910 in the tail of its X-ray diffuse emission. We suggest that these objects originated from a binary system with the rotational and radial velocities of VFTS102 resulting from mass transfer from the progenitor of PSR J0537-691 and the supernova explosion respectively.

astro-ph.SR↗
arXiv2011

The VLT-FLAMES Tarantula Survey I: Introduction and observational overview

The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our scientific motivations and give an overview of the survey targets, including optical and near-infrared photometry and comprehensive details of the data reduction. One of the principal objectives was to detect massive binary systems via variations in their radial velocities, thus shaping the multi-epoch observing strategy. Spectral classifications are given for the massive emission-line stars observed by the survey, including the discovery of a new Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To illustrate the diversity of objects encompassed by the survey, we investigate the spectral properties of sixteen targets identified by Gruendl & Chu from Spitzer photometry as candidate young stellar objects or stars with notable mid-infrared excesses. Detailed spectral classification and quantitative analysis of the O- and B-type stars in the VFTS sample, paying particular attention to the effects of rotational mixing and binarity, will be presented in a series of future articles to address fundamental questions in both stellar and cluster evolution.

astro-ph.SR↗
arXiv2011

The VLT-FLAMES Tarantula Survey II: R139 revealed as a massive binary system

We report the discovery that R139 in 30 Doradus is a massive spectroscopic binary system. Multi-epoch optical spectroscopy of R139 was obtained as part of the VLT-FLAMES Tarantula Survey, revealing a double-lined system. The two components are of similar spectral types; the primary exhibits strong C III 4650 emission and is classified as an O6.5 Iafc supergiant, while the secondary is an O6 Iaf supergiant. The radial-velocity variations indicate a highly eccentric orbit with a period of 153.9 days. Photometry obtained with the Faulkes Telescope South shows no evidence for significant variability within an 18 month period. The orbital solution yields lower mass limits for the components of M1sin^3 i = 78 \pm 8 Msun and M2sin^3 i = 66 \pm 7 Msun. As R139 appears to be the most massive binary system known to contain two evolved Of supergiants, it will provide an excellent test for atmospheric and evolutionary models.

astro-ph.SR↗
arXiv2011

Spectroscopic and physical parameters of Galactic O-type stars. I. Effects of rotation and spectral resolving power in the spectral classification of dwarfs and giants

The modern-era spectral classification of O-stars relies on either the Walborn or the Conti-Mathys scheme. Since both of these approaches have been developed using low-quality photographic data, their application to high-quality digital data might not be straightforward and be hampered by problems and complications that have not yet been appreciated. Using high-resolution spectra obtained with the ESO/MPG 2.2\,m telescope in La Silla and following the premises of the Walborn and Conti classification schemes, we determined the spectral types and luminosity classes of 19 Galactic O-type stars and compared them to those attributed by Walborn and Mathys based on low-quality data. Our analysis reveals that the morphological spectral types assigned using high-resolution data are systematically later (by up to 1.5 subtypes) then those attributed by Walborn. By means of line-profile simulations, we show that part of this discrepancy is more likely caused by the combined effect of stellar rotation and high spectral resolution on the depth of helium lines used as spectral type indicators. In addition, we demonstrate that at least for narrow-lined stars the "rotational effect" does not disappear when the high-resolution spectra are degraded to the resolution of the Walborn standards. We also find evidence of a systematic difference between our high-resolution quantitative spectral types and those assigned by Mathys. Rotation and spectral resolution are important third parameters in the spectral classification of O-type stars. To obtain reliable spectral classes within the Walborn approach, the unknown and the standard spectra must be compared at the same resolution and \vsini. Owing to resolution effects, the Conti approach might also need to be updated.

astro-ph.GA↗
arXiv2011

The FLAMES Tarantula Survey

The Tarantula survey is an ESO Large Programme which has obtained multi-epochs spectroscopy of over 800 massive stars in the 30 Dor region in the Large Magelanic Cloud. Here we briefly describe the main drivers of the survey and the observational material derived.

astro-ph.CO↗
arXiv2011

The H-alpha Variations of the Luminous Blue Variable P Cygni: Discrete Absorption Components and the Short S Doradus Phase

P Cygni is a prototype of the Luminous Blue Variables (or S Doradus variables), and the star displays photometric and emission line variability on a timescale of years (known as the "short S Doradus phase" variations). Here we present new high resolution H-alpha spectroscopy of P Cyg that we combine with earlier spectra and concurrent V-band photometry to document the emission and continuum flux variations over a 24 y time span. We show that the emission and continuum fluxes vary in concert on timescales of 1.6 y and longer, but differ on shorter timescales. The H-alpha profile shape also varies on the photometric timescales, and we describe the observed co-variations of the emission peak and absorption trough properties. We argue that the episodes of photometric and emission brightening are caused by increases in the size of the emission region that are related to variations in wind mass loss rate and outflow speed. We find evidence of blueward accelerating, Discrete Absorption Components (DACs) in the absorption trough of the H-alpha profile, and these features have slower accelerations and longer durations than those observed in other lines. The DAC strengths also appear to vary on the photometric timescales, and we suggest that the propagation of the DAC-related wind structures is closely related to changes in the overall wind mass loss rate and velocity.

astro-ph.SR↗
arXiv2010

The IACOB spectroscopic database of Northern Galactic OB stars

We present the IACOB spectroscopic database, an homogeneous set of high quality, high resolution spectra of Galactic O- and B-type stars obtained with the FIES spectrograph attached to the Nordic Optical Telescope. We also present some results from ongoing projects using the IACOB database.

astro-ph.SR↗
arXiv2009

The VLT-FLAMES Tarantula Survey

The Tarantula Survey is an ambitious ESO Large Programme that has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. Here we introduce the scientific motivations of the survey and give an overview of the observational sample. Ultimately, quantitative analysis of every star, paying particular attention to the effects of rotational mixing and binarity, will be used to address fundamental questions in both stellar and cluster evolution.

astro-ph.CO↗
arXiv2009

On the model of a classical relativistic particle of constant and universal mass and spin

The deformation of the classical action for a free relativistic particle recently suggested by A. Staruszkiewicz gives rise to a spin structure which constrains the values of the invariant mass and the invariant spin to be the same for any solution of the equations of motion. We prove that both these Casimir invariants, the square of the four-momentum vector and the square of the Pauli-Lubański pseudo-vector, preserve the same fixed values even in the presence of an arbitrary external electromagnetic field. In the "free" case, in the centre of mass reference frame, the particle moves along a circle of fixed radius. In a homogeneous magnetic field, a number of rotational "states" is possible with frequencies slightly different from the cyclotron frequency, and "phase-like" transitions with spin flops occure at some critical value of the particle's three-momentum. In the last section, the article of Kuzenko, Lyakhovich and Segal (1994) in which, in fact, an equivalent model had been proposed and elaborated, is briefly reviewed and compared with Staruszkiewicz's approach and our results.

hep-th↗
arXiv2009

Wind structure in late-B supergiants

Extended spectroscopic datasets of several late-B stars of luminosity class Ia revealed the presence of similar peculiarities in their H alpha profiles, which might be interpreted as indications of deviation from spherically symmetric, smooth wind approximation. Surface structures due to non-radial pulsations or weak, large-scale, dipole magnetic fields might be responsible for creating wind structure in the envelopes of these stars.

astro-ph.SR↗
arXiv2009

Metallicity effects in the spectral classification of O-type stars. Theoretical consideration

Based on an exteded grid of NLTE, line blanketed model atmospheres with stellar winds as calculated by means of FASTWIND, we have investigated the change in the strengths of strategic Helium transitions in the optical as caused by a 0.3 decrease in metallicity with respect to solar abundances. Our calculations predict that only part of the observed increase in Teff, of O-type dwarfs could be explained by metallicity effects on the spectral type indicators, while the rest must be attributed to other reasons (e.g., different stellar structures as a function of metallicity or differences between observed and theoretical wind parameters etc.). In addition, we found that using the He II 4686 line to classify stars in low metallicity environments (Z < 0.3 solar) might artificially increase the number of low luminosity (dwarfs and giants) O-stars, on the expense of the number of O-supergiants.

astro-ph.CO↗
arXiv2008

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.

astro-ph↗
arXiv2007

Bright OB stars in the Galaxy IV. Stellar and wind parameters of early to late B supergiants

We apply the NLTE atmosphere code FASTWIND to perform a spectroscopic study of a small sample of Galactic B-supergiants from B0 to B9. By means of the resulting data and incorporating additional datasets from alternative studies, we investigate the properties of OB-supergiants and compare our findings with theoretical predictions. As a result we find that due to the combined effects of line- and wind-blanketing, the temperature scale of Galactic B-supergiants needs to be revised downwards, by 10 to 20 percent, the latter value being appropriate for stronger winds. In fair accordance with recent results, our sample furthermore indicates a gradual decrease in wind terminal velocities over the bi-stability region, where the limits of this region are located at lower temperatures than the predicted ones. Introducing a distance-independent quantity Q' related to wind-strength, we also show that this quantity is a well defined, monotonically increasing function of Teff outside this region. Inside and from hot to cool, the mass loss rate changes by a factor (in between 0.4 and 2.5) which is (much) smaller than the predicted factor of 5. All this indicates that the decrease in wind terminal velocity over the bi-stability region is not over-compensated by an increase of mass loss rate, as frequently argued (provided the wind-clumping properties on both sides of this region do not differ substantially).

astro-ph↗
arXiv2006

Stellar winds from massive stars - What are the REAL mass-loss rates?

We discuss recent evidence that currently accepted mass-loss rates may need to be revised downwards, as a consequence of previously neglected ``clumping'' of the wind. New results on the radial stratification of the corresponding clumping factors are summarized. We investigate the influence of clumping on the ionization equilibrium of phosphorus, which is of major relevance when deriving constraints on the clumping factors from an analysis of the FUV PV resonance lines.

astro-ph↗
arXiv2006

The superimposed photospheric and stellar wind variability of the O-type supergiant alpha Cam

(Abridged) We provide empirical constraints on the different physical components that can act to yield temporal variability in predominantly or partially wind-formed optical lines of luminous OB stars, and thus potentially affect the reliable determination of fundamental parameters, including mass-loss rates via clumped winds. Using time-series spectroscopy from epochs spread over $\sim$ 4 years, a case study of the O9.5 supergiant $α$ Cam is presented. We demonstrate that the He I 5876 (2$^3$P$^0$--3$^3$D) line is an important diagnostic for photospheric and wind variability in this star. The optical line profiles of $α$ Cam are affected by (i) deep-seated fluctuations close to, or at, the photosphere, (ii) atmospheric velocity gradients, and (iii) large-scale stellar wind structure. This study provides new empirical perspectives on accurate line-synthesis modelling of stellar wind signatures in massive luminous stars. Using a pure Halpha line-synthesis code we interpret maximum changes in the red-ward and peak emission of $α$ Cam in terms of mass-loss rate differences in the range $\sim$ 5.1 $\times$ 10$^{-6}$ to 6.5 $\times$ 10$^{-6}$ M$_\odot$ yr$^{-1}$. However, the models generally fail to reproduce the morphology of blueward (possibly absorptive) regions of the profiles.

astro-ph↗