The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial ...mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses (Formula: see text). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 Formula: see text and contains 32 ± 12% more stars above 30 Formula: see text than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 Formula: see text, the IMF power-law exponent is Formula: see text, shallower than the Salpeter value of 2.35.
Context. The nuclear star cluster of the Galaxy is an important template for understanding its extragalactic counterparts, which can currently not be resolved into individual stars. Important ...drawbacks of observations of the Galactic center are, however, the presence of strong and spatially highly variable interstellar extinction and extreme crowding of the sources, which makes the use of adaptive optics techniques necessary. Both points pose serious obstacles to precise photometry that is needed for analyzing the stellar population. Aims. The aims of this work are to provide accurate photometry in multiple near-infrared broadband filters, to determine the power-law index of the extinction-law toward the central parsec of the Galaxy, to provide measurements of the absolute extinction toward the Galactic center, and finally to measure the spatial variability of extinction on arcsecond scales. Methods. We use observations of the central parsec of the Milky Way that were obtained with the near-infrared camera and adaptive optics system NAOS/CONICA at the ESO VLT unit telescope 4. The photometric method takes into account anisoplanatic effects and limits the corresponding systematic uncertainties to ≲2%. Absolute values for the extinction in the H, $K{\rm s}$, and L'-bands as well as of the power-law indices of the H to $K{\rm s}$ and $K{\rm s}$ to L' extinction-laws are measured based on the well-known properties of red clump stars. Extinction maps are derived based on $H-K{\rm s}$ and $K{\rm s}-L'$ colors. Results. We present $K{\rm s}$-band photometry for ~7700 stars, and additionally photometry for stars detected in the H and/or L'-bands. From a number of recently published values we compute a mean distance of the Galactic center of $R_{0}=8.03\pm0.15$ kpc, which has an uncertainty of just $2\%$. Based on this $R_{0}$ and on the RC method, we derive absolute mean extinction values toward the central parsec of the Galaxy of $A_{H}=4.48\pm0.13$ mag, $A_{K{\rm s}}=2.54\pm0.12$ mag, and $A_{L'}=1.27\pm0.18$ mag. We estimate values of the power-law indices of the extinction-law of $\alpha_{H-K{\rm s}}=2.21\pm0.24$ and $\alpha_{K{\rm s}-L'}=1.34\pm0.29$. A $K{\rm s}$-band extinction map for the Galactic center is computed based on this extinction law and on stellar $H-K{\rm s}$ colors. Both its statistical and systematic uncertainties are estimated to be <10%. Extinction in this map derived from stellar color excesses is found to vary on arcsecond scales, with a mean value of $A_{K{\rm s}}=2.74\pm0.30$ mag. Mean extinction values in a circular region with 0.5'' radius centered on Sagittarius A* are $A_{H, {\rm SgrA*}}=4.35\pm0.12$, $A_{K{\rm s}, {\rm SgrA*}}=2.46\pm0.03$, and $A_{L', {\rm SgrA*}}=1.23\pm0.08$.
Context. Mass-loss, occurring through radiation driven supersonic winds, is a key issue throughout the evolution of massive stars. Two outstanding problems are currently challenging the theory of ...radiation-driven winds: wind clumping and the weak-wind problem. Aims. We seek to obtain accurate mass-loss rates of OB stars at different evolutionary stages to constrain the impact of both problems in our current understanding of massive star winds. Methods. We perform a multi-wavelength quantitative analysis of a sample of ten Galactic OB-stars by means of the atmospheric code cmfgen, with special emphasis on the L-band window. A detailed investigation is carried out on the potential of Brα and Pfγ as mass-loss and clumping diagnostics. Results. For objects with dense winds, Brα samples the intermediate wind while Pfγ maps the inner one. In combination with other indicators (UV, Hα, Brγ) these lines enable us to constrain the wind clumping structure and to obtain “true” mass-loss rates. For objects with weak winds, Brα emerges as a reliable diagnostic tool to constrain Ṁ. The emission component at the line Doppler-core superimposed on the rather shallow Stark absorption wings reacts very sensitively to mass loss already at very low Ṁ values. On the other hand, the line wings display similar sensitivity to mass loss as Hα, the classical optical mass loss diagnostics. Conclusions. Our investigation reveals the great diagnostic potential of L-band spectroscopy to derive clumping properties and mass-loss rates of hot star winds. We are confident that Brα will become the primary diagnostic tool to measure very low mass-loss rates with unprecedented accuracy.
Context. The study of the inner region of the Milky Way bulge is hampered by high interstellar extinction and extreme source crowding. Sensitive high angular resolution near-infrared imaging is ...needed to study stellar populations and their characteristics in such a dense and complex environment. Aims. We aim at investigating the stellar population in the innermost Galactic bulge, to study the star formation history in this region of the Galaxy. Methods. We used the 0.2″ angular resolution JHKs data from the GALACTICNUCLEUS survey to study the stellar population within two 8.0′×3.4′ fields, about 0.6° and 0.4° to the Galactic north of the Milky Way centre and to compare it with the one in the immediate surroundings of Sagittarius A*. We also characterise the absolute extinction and the extinction curve of the two fields. Results. The average interstellar extinction to the outer and the inner field is AKs ∼ 1.20 ± 0.08 mag and ∼1.48 ± 0.10 mag, respectively. We present Ks luminosity functions that are complete down to at least two magnitudes below the red clump (RC). We detect a feature in the luminosity functions that is fainter than the RC by 0.80 ± 0.03 and 0.79 ± 0.02 mag, respectively, in the Ks band. It runs parallel to the reddening vector. We identify the feature as the red giant branch bump. Fitting α-enhanced BaSTI luminosity functions to our data, we find that a single old stellar population of ∼12.8 ± 0.6 Gyr and Z = 0.040 ± 0.003 provides the best fit. Our findings thus show that the stellar population in the innermost bulge is old, similar to the one at larger distances from the Galactic plane, and that its metallicity is about twice solar at distances as short as about 60 pc from the centre of the Milky Way, similar to what is observed at about 500 pc from the Galactic Centre. Comparing the obtained metallicity with previous known values at larger latitudes (|b| > 2°), our results favour a flattening of the gradient at |b| < 2°. As a secondary result we obtain that the extinction index in the studied regions agrees within the uncertainties with our previous value of α = 2.30 ± 0.08 that was derived for the very Galactic centre.
ABSTRACT
Local Group (LG) very metal-poor massive stars are the best proxy for the First Stars of the Universe and fundamental to modelling the evolution of early galaxies. These stars may follow new ...evolutionary pathways restricted to very low metallicities, such as chemically homogeneous evolution (CHE). However, given the great distance leap needed to reach very metal-poor galaxies of the LG and vicinity, no comprehensive spectroscopic studies have been carried out at metallicities lower than the Small Magellanic Cloud (SMC; Z = 1/5 Z⊙) until now. After five observing campaigns at the 10.4-m Gran Telescopio Canarias, we have assembled a low-resolution (R ∼ 1000) spectroscopic collection of more than 150 OB stars in the 1/10 Z⊙ galaxy Sextans A, increasing by an order of magnitude the number of massive stars known in this galaxy. The catalogue includes 38 BA-type supergiants, four red supergiants, and the first candidate 1/10 Z⊙ binary systems, CHE sources and systems hosting stripped stars. The sample massive stars mainly overlap the higher concentrations of neutral gas of Sextans A. However, we find some sources in low H i column-density regions. The colour–magnitude diagram of the galaxy presents large dispersion, which suggests uneven, internal extinction in Sextans A. This is the largest catalogue of OB-type stars ever produced at sub-SMC metallicities. This sample constitutes a fundamental first step to unveiling the evolutionary pathways and fates of very metal-poor massive stars, analyzing the dependence of radiation-driven winds with metallicity, and studying binary systems in an environment analogue to the early Universe.
Context. Due to the extreme extinction towards the Galactic centre (AV ∼ 30 mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR ...extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. Aims. We studied the dependence of the extinction index (αλ) in the NIR on the line of sight, wavelength, and extinction. Methods. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue (0.2″) for the inner part of the Galaxy in JHKs, and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. Results. We did not detect any significant line-of-sight or extinction variation in α within the studied region in the nuclear stellar disc. The extinction index between JH and HKs differs by 0.19 ± 0.05. We obtained mean values for the extinction indices αJH = 2.43 ± 0.03 and αHKs = 2.23 ± 0.03. The dependence of the extinction index on the wavelength could explain the differences obtained for αλ in the literature since it was assumed constant for the NIR regime.
Context.
Many galaxies host pronounced circumnuclear starbursts, fuelled by infalling gas. Such activity is expected to drive the secular evolution of the nucleus and generate super winds which ...enrich the interstellar and intergalactic medium. Moreover, given the intense radiation fields and extreme gas and cosmic ray densities present within such regions, one might question whether star formation proceeds in a different manner to that occurring in more quiescent regions of the galactic disc, potentially leading to a dependence of the stellar initial mass function on the local environment.
Aims.
To address the physics of circumnuclear starbursts, we are driven to observe the centre of our own Galaxy, which is the only example where individual stars may be resolved. Previous studies have revealed a rich population of very massive stars, found in three young massive clusters as well as being distributed, in apparent isolation, throughout the inner ∼500 pc of the Galaxy. In this paper we investigate the size and composition of the latter cohort in order to constrain its origin and role in the lifecycle of the Galactic Centre.
Methods.
To accomplish this, we utilised the Very Large Telescope +
K
-band Multi-Object Spectrograph to obtain homogeneous, high signal-to-noise ratio observations of known and candidate massive stars suitable for spectral classification and quantitative analysis.
Results.
We identified 17 new isolated massive stars and reclassified a further 19 known examples, leading to a total of at least 83 within the Galactic Centre. Due to the selection criteria employed, these were strongly biased towards stars with powerful stellar winds and/or extensive circumstellar envelopes; as such, we suspect the resultant census to be incomplete. No further stellar clusters, or their tidally stripped remnants, were identified, although an apparent overdensity of very young and massive stars is found to be coincident with the Sgr B1 star forming region.
Conclusions.
Despite the limitations of the current dataset, the size of the cohort of outlying massive stars within the Galactic Centre is directly comparable to that of the known clusters and, assuming a comparable mass function, is expected to exceed this number. Combining both cluster and isolated populations yields ≳320 spectroscopically classified stars within the Galactic Centre that are sufficiently massive that they might be anticipated to undergo core collapse within the next ∼20 Myr. Given this is almost certainly a substantial underestimate of the true number, the population of massive stars associated with the Galactic Centre appears unprecedented within the Milky Way, and it appears unavoidable that they play a substantial role in the energetics and evolution of this region.
ABSTRACT
Serving as the progenitors of electromagnetic and gravitational-wave transients, massive stars have received renewed interest in recent years. However, many aspects of their birth and ...evolution remain opaque, particularly in the context of binary interactions. The centre of our Galaxy hosts a rich cohort of very massive stars, which appear to play a prominent role in the ecology of the region. In this paper, we investigate the binary properties of the Arches cluster, which is thought to host a large number of very massive stars. A combination of multi-epoch near-IR spectroscopy and photometry was utilized to identify binaries. 13 from 36 cluster members meet our criteria to be classed as RV variable. Combining the spectroscopic data with archival radio and X-ray observations – to detect colliding wind systems – provides a lower limit to the binary fraction of ∼43 per cent; increasing to ≳50 per cent for the O-type hypergiants and WNLha. Dynamical and evolutionary masses reveal the primaries to be uniformly massive ($\gtrsim 50\, \mathrm{M}_{\odot }$). Where available, orbital analysis reveals a number of short period, highly eccentric binaries, which appear to be pre-interaction systems. Such systems are X-ray luminous, with 80 per cent above an empirical bound of (Lx/Lbol) ∼ 10−7 and their orbital configurations suggest formation and evolution via a single-star channel; however, we cannot exclude a binary formation channel for a subset. Qualitative comparison to surveys of lower mass OB-type stars confirms that the trend to an extreme binary fraction (≥60 per cent) extends to the most massive stars currently forming in the local Universe.
We briefly describe the results of a K-band spectroscopic survey of over 500 highly reddened point-like objects on sightlines toward the Central Molecular Zone (CMZ) of the Galaxy. The goal was to ...find stars with featureless or nearly featureless spectra suitable for near- and mid-infrared absorption spectroscopy of the Galactic center's interstellar gas on sightlines spread across the CMZ. Until recently only a few such stars have been known outside of very localized sightlines in the vicinity of the Quintuplet and Central clusters. We have used Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire and Two-Micron All-Sky Survey photometry to select promising candidates, and over the last 10 years have been acquiring low-resolution K-band spectra of them. As expected, the vast majority are cool and/or highly reddened red giants with complex photospheric spectra unsuitable for measuring faint interstellar lines. Approximately 10% of them, whose observations are reported here, have featureless or nearly featureless spectra. Although not evenly distributed in Galactic longitude, these stars are scattered across the CMZ. Many of them are luminous stars that are deeply embedded in warm dust cocoons, and have K-band continua rising steeply to longer wavelengths. A significant fraction of them are hot stars of a variety of spectral types, including at least five newly discovered Wolf-Rayet stars. All of them should be suitable for spectroscopy of interstellar absorption lines at infrared wavelengths greater than 3 m and many are also suitable at shorter wavelengths.
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