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Register a new userTesting massive star evolution, star-formation history, and feedback at low metallicity: Photometric analysis of OB stars in the SMC Wing
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The supergiant ionized shell SMC-SGS 1 (DEM 167), located in the outer Wing of the Small Magellanic Cloud (SMC), resembles structures that originate from an energetic star-formation event and later stimulate star formation as they expand into the ambient medium. However, stellar populations within and surrounding SMC-SGS 1 tell a different story. We present a photometric study of the stellar population encompassed by SMC-SGS 1 in order to trace the history of this structure and its potential influence on star formation within the low-density, low-metallicity SMC Wing. For a stellar population that is physically associated with SMC-SGS 1, we combined near-ultraviolet (NUV) photometry from the Galaxy Evolution Explorer (GALEX) with archival optical (V-band) photometry from the ESO Danish 1.54m Telescope. Given their colors and luminosities, we estimated stellar ages and masses by matching observed photometry to theoretical stellar isochrone models. We find that the investigated region supports an active, extended star-formation event spanning $sim$ 25 - 40 Myr ago, as well as continued star formation into the present. Using a standard initial mass function (IMF), we infer a lower bound on the stellar mass from this period of $sim 3 times 10^4 M_{odot}$, corresponding to a star-formation intensity of $sim$ 6 $times$ 10$^{-3}$ M$_{odot}$ kpc$^{-2}$ yr$^{-1}$. The spatial and temporal distributions of young stars encompassed by SMC-SGS 1 imply a slow, consistent progression of star formation over millions of years. Ongoing star formation along the edge of and interior to SMC-SGS 1 suggests a combined stimulated and stochastic mode of star formation within the SMC Wing. A slow expansion of the shell within this low-density environment may preserve molecular clouds within the volume of the shell, leaving them to form stars even after nearby stellar feedback expels local gas and dust.
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Stars which start their lives with spectral types O and early-B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 OB stars in the Small Magellanic Cloud. The data, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models. We find that stellar winds of our sample stars are much weaker than theoretically expected. The stellar rotation rates show a bi-modal distribution. The well-populated upper Hertzsprung-Russell diagram including our sample OB stars from SMC Wing as well as additional evolved stars all over SMC from the literature shows a strict luminosity limit. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with Minit<30M$_{odot}$ seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with Minit>30M$_{odot}$ appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We report extended star-formation episodes in a quiescent low-density region of the Wing, which is progressing stochastically. We measure the key parameters of stellar feedback and establish the links between the rates of star formation and supernovae. Our study reveals that in metal-poor environments the stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars.
The rich SMC star cluster NGC419 has recently been found to present both a broad main sequence turn-off and a dual red clump of giants, in the sharp colour-magnitude diagrams (CMD) derived from the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. In this work, we apply to the NGC419 data the classical method of star formation history (SFH) recovery via CMD reconstruction, deriving for the first time this function for a star cluster with multiple turn-offs. The values for the cluster metallicity, reddening, distance and binary fraction, were varied within the limits allowed by present observations. The global best-fitting solution is an excellent fit to the data, reproducing all the CMD features with striking accuracy. The corresponding star formation rate is provided together with estimates of its random and systematic errors. Star formation is found to last for at least 700 Myr, and to have a marked peak at the middle of this interval, for an age of 1.5 Gyr. Our findings argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1 Gyr. It remains to be tested whether alternative hypotheses, such as a main sequence spread caused by rotation, could produce similarly good fits to the data.
Deep HST/ACS photometry of the young cluster NGC 602, located in the remote low density wing of the Small Magellanic Cloud, reveals numerous pre-main sequence stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history into recent times and constraining several stellar population properties, such as the present day mass function, the initial mass function and the binary fraction. To better characterize the pre-main sequence population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the star formation history in the region by comparing observed and synthetic color-magnitude diagrams. The derived present day mass function for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching $0.3-0.7 times 10^{-3} M_odot yr^{-1}$ in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.
Clusters or associations of early-type stars are often associated with a superbubble of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N206 in the Large Magellanic Cloud exhibits a superbubble and a rich massive star population. We observed these massive stars using the FLAMES multi-object spectrograph at ESO-VLT. Available UV spectra from HST, IUE, and FUSE are also used. The spectral analysis is performed with Potsdam Wolf-Rayet (PoWR) model atmospheres. We present the stellar and wind parameters of the OB stars and the two WR binaries in the N206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The HRD of the OB stars reveals a large age spread (1-30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N206 complex as well as for the cluster NGC2018. Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is ~$2.3times10^{52}$ erg, which exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble.
We study the evolution, rotation, and surface abundances of O-type dwarfs in the Small Magellanic Cloud. We analyzed the UV and optical spectra of twenty-three objects and derived photospheric and wind properties. The observed binary fraction of the sample is ~ 26%, which is compatible with more systematic studies, if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample excluded obvious spectroscopic binaries. The location of the fastest rotators in the H-R diagram indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity-class Vz, expected to best characterize extreme youth, is located on the ZAMS, the other two stars are more evolved. The distribution of nitrogen abundance of O and B stars suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depends only weakly on the stars mass. We confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that matches the predictions of the stellar evolution models well. More massive stars have a higher N/C ratio than the less massive stars. Faster rotators show on average a higher N/C ratio than slower rotators. The N/O versus N/C ratios agree qualitatively well with those of stellar evolution models. The only discrepant behavior is observed for the youngest two stars of the sample, which both show very strong signs of mixing, which is unexpected for their evolutionary status.
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