- ID:
- ivo://CDS.VizieR/J/A+A/625/A132
- Title:
- Models for massive stars in the SMC
- Short Name:
- J/A+A/625/A132
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The evolution of massive stars is strongly influenced by internal mixing processes such as semiconvection, convective core overshooting, and rotationally induced mixing. None of these processes are currently well constrained. We investigate models for massive stars in the Small Magellanic Cloud (SMC), for which stellar-wind mass loss is less important than for their metal-rich counterparts. We aim to constrain the various mixing efficiencies by comparing model results to observations. For this purpose, we use the stellar-evolution code MESA to compute more than 60 grids of detailed evolutionary models for stars with initial masses of 9...100M_{sun}_, assuming different combinations of mixing efficiencies of the various processes in each grid. Our models evolve through core hydrogen and helium burning, such that they can be compared with the massive main sequence and supergiant population of the SMC. We find that for most of the combinations of the mixing efficiencies, models in a wide mass range spend core-helium burning either only as blue supergiants, or only as red supergiants. The latter case corresponds to models that maintain a shallow slope of the hydrogen/helium (H/He) gradient separating the core and the envelope of the models. Only a small part of the mixing parameter space leads to models that produce a significant number of blue and red supergiants, which are both in abundance in the SMC. Some of our grids also predict a cut-o in the number of red supergiants above logL/L_{sun}_=5...5.5. Interestingly, these models contain steep H/He gradients, as is required to understand the hot, hydrogen-rich Wolf-Rayet stars in the SMC. We find that unless it is very fast, rotation has a limited effect on the H/He profiles in our models. While we use specific implementations of the considered mixing processes, they comprehensively probe the two firstorder structural parameters, the core mass and the H/He gradient in the core-envelope interface. Our results imply that in massive stars, mixing during the main-sequence evolution leads to a moderate increase in the helium core masses, and also that the H/He gradients above the helium cores become very steep. Our model grids can be used to further refine the various mixing efficiencies with the help of future observational surveys of the massive stars in the SMC, and thereby help to considerably reduce the uncertainties in models of massive star evolution.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/600/A93
- Title:
- Multiwavelength fluxes of RCW120
- Short Name:
- J/A+A/600/A93
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The expansion of HII regions can trigger the formation of stars. An overdensity of young stellar objects (YSOs) is observed at the edges of HII regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between HII-region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering. We have observed the Galactic HII region RCW120 with Herschel PACS and SPIRE photometers at 70, 100, 160, 250, 350 and 500um. We produced temperature and H_2_ column density maps and use the getsources algorithm to detect compact sources and measure their fluxes at Herschel wavelengths. We have complemented these fluxes with existing infrared data. Fitting their spectral energy distributions (SEDs) with a modified blackbody model, we derived their envelope dust temperature and envelope mass. We computed their bolometric luminosities and discuss their evolutionary stages. The Herschel data, with their unique sampling of the far infrared domain, have allowed us to characterize the properties of compact sources observed towards RCW120 for the first time. We have also been able to determine the envelope temperature, envelope mass and evolutionary stage of these sources. Using these properties we have shown that the density of the condensations that host star formation is a key parameter of the star-formation history, irrespective of their projected distance to the ionizing stars.
- ID:
- ivo://CDS.VizieR/II/26A
- Title:
- Narrow-band photometry of early-type stars
- Short Name:
- II/26A
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A compilation of the photoelectric measurements in the Barbier-Morguleff system is presented for 773 stars of spectral types O8 to F6. 706 stars have been measured at least twice. The passbands (in Angstroems) and the mean photometric errors of the indexes C(j)-C(2) (in magnitudes) are: ----------------------- Band Interval m.e. ----------------------- C(1) 6000-5900 0.0318 C(2) 5000-4900 --- C(3) 4905-4815 0.0073 C(4) 4380-4310 0.0100 C(5) 4070-4010 0.0102 C(6) 3965-3925 0.0201 C(7) 3780-3720 0.0195 C(8) 3650-3575 0.0189 ----------------------- Regarding the catalog Pass Bands Morguleff and Gerbaldi (1975) note the following: These spectral intervals permit the determination of certain physical characteristics .... The pass bands 1 and 5 lead us to calculate red and blue gradients. The equivalent widths of the H(beta), H(gamma) lines of hydrogen and the K lines of Ca II are derived from band 3, 4, and 6. Bands 7 and 8 measure the Balmer jump. Listed in the catalog file are the normalized magnitudes corresponding to the above pass bands. The normalized magnitude of a pass band, C(j), is defined as the color index of the magnitude in the band minus the magnitude of the second band, C(2). See Morguleff and Gerbaldi (1975) for a brief discussion of conservation of the system and mean errors across all stars.
- ID:
- ivo://CDS.VizieR/J/A+A/487/307
- Title:
- Nearby B-type stars abundances
- Short Name:
- J/A+A/487/307
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The recent downward revision of the solar photospheric abundances now leads to severe inconsistencies between the theoretical predictions for the internal structure of the Sun and the results of helioseismology. There have been claims that the solar neon abundance may be underestimated and that an increase in this poorly-known quantity could alleviate (or even completely solve) this problem. Early-type stars in the solar neighbourhood are well-suited to testing this hypothesis because they are the only stellar objects whose absolute neon abundance can be derived from the direct analysis of photospheric lines. Here we present a fully homogeneous NLTE abundance study of the optical Ne I and Ne II lines in a sample of 18 nearby, early B-type stars, which suggests log[epsilon(Ne)]=7.97+/-0.07dex (on the scale in which log[epsilon(H)=12)]) for the present-day neon abundance of the local ISM. Chemical evolution models of the Galaxy only predict a very small enrichment of the nearby interstellar gas in neon over the past 4.6Gyr, implying that our estimate should be representative of the Sun at birth. Although higher by about 35% than the new recommended solar abundance, such a value appears insufficient by itself to restore the past agreement between the solar models and the helioseismological constraints.
- ID:
- ivo://CDS.VizieR/J/ApJS/161/154
- Title:
- Near-IR spectral atlas of OB stars
- Short Name:
- J/ApJS/161/154
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present intermediate-resolution (R~8000-12000) high signal-to-noise (S/N) H- and K-band spectroscopy of a sample of 37 optically visible stars, ranging in spectral type from O3 to B3 and representing most luminosity classes. Spectra of this quality can be used to constrain the temperature, luminosity, and general wind properties of OB stars, when used in conjunction with sophisticated atmospheric model codes. Most important is the need for moderately high resolutions (R>=5000) and very high signal-to-noise (S/N>=150) spectra for a meaningful profile analysis. When using near-infrared spectra for a classification system, moderately high signal-to-noise (S/N~100) is still required, though the resolution can be relaxed to just a thousand or two. In the Appendix we provide a set of very high-quality near-infrared spectra of Brackett lines in six early-A dwarfs. These can be used to aid in the modeling and removal of such lines when early-A dwarfs are used for telluric spectroscopic standards.
- ID:
- ivo://CDS.VizieR/J/A+A/533/A70
- Title:
- New {beta} Cep stars in NGC 637
- Short Name:
- J/A+A/533/A70
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Studying stellar pulsations in open clusters offers the possibility of performing ensemble asteroseismology. The reasonable assumption that the cluster members have the same age, distance, and overall metallicity aids in the seismic modelling process and tightly constrains it. Therefore it is important to identify open clusters with many pulsators. New pulsating stars of the beta Cephei type were searched for among the members of the open cluster NGC 637. Thirty-one hours of time-resolved V filter CCD photometry were obtained. The measurements confirmed two previously known variables, and revealed three new beta Cephei stars and one more candidate. All four pulsators have sufficiently large amplitudes for easy mode identification and are multiperiodic. With four certain pulsating members, NGC 637 is now among the open clusters that are richest in beta Cephei stars. It is thus an excellent target for ensemble asteroseismology, and to examine the differences between pulsating and apparently constant stars in the beta Cephei domain.
- ID:
- ivo://CDS.VizieR/J/MNRAS/457/3396
- Title:
- New hot subdwarf stars
- Short Name:
- J/MNRAS/457/3396
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Recent massive sky surveys in different bandwidths are providing new opportunities to modern astronomy. The Virtual Observatory (VO) represents the adequate framework to handle the huge amount of information available and filter out data according to specific requirements. In this work, we applied a selection strategy to find new, uncatalogued hot subdwarfs making use of VO tools. We used large area catalogues like GALEX, Sloan Digital Sky Survey (SDSS), SuperCosmos and Two Micron All Sky Survey (2MASS) to retrieve photometric and astrometric information of stellar objects. To these objects, we applied colour and proper motion filters, together with an effective temperature cutoff, aimed at separating hot subdwarfs from other blue objects such as white dwarfs, cataclysmic variables or main-sequence OB stars. As a result, we obtained 437 new, uncatalogued hot subdwarf candidates. Based on previous results, we expect our procedure to have an overall efficiency of at least 80 per cent. Visual inspection of the 68 candidates with SDSS spectrum showed that 65 can be classified as hot subdwarfs: 5 sdOs, 25 sdOBs and 35 sdBs. This success rate above 95 per cent proves the robustness and efficiency of our methodology. The spectral energy distribution of 45 per cent of the subdwarf candidates showed infrared excesses, a signature of their probable binary nature. The stellar companions of the binary systems so detected are expected to be late-type main-sequence stars. A detailed determination of temperatures and spectral classification of the cool companions will be presented in a forthcoming work.
- ID:
- ivo://CDS.VizieR/J/A+A/530/A2
- Title:
- New hot subdwarf stars
- Short Name:
- J/A+A/530/A2
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Recent massive sky surveys in different bandwidths are providing new opportunities to modern astronomy. The Virtual Observatory (VO) provides the adequate framework to handle the huge amount of information available and filter out data according to specific requirements. Hot subdwarf stars are faint, blue objects, and are the main contributors to the far-UV excess observed in elliptical galaxies. They offer an excellent laboratory to study close and wide binary systems, and to scrutinize their interiors through asteroseismology, since some of them undergo stellar oscillations. However, their origins are still uncertain, and increasing the number of detections is crucial to undertake statistical studies. In this work, we aim at defining a strategy to find new, uncatalogued hot subdwarfs.
- ID:
- ivo://CDS.VizieR/J/PAZh/21/13
- Title:
- New list of OB associations
- Short Name:
- J/PAZh/21/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A new partition into associations of OB stars within 3kpc of the Sun is derived using Battinelli's modification of the cluster analysis method. We have found 58 associations, of which only 10% may be considered as a random clusters of field stars.
- ID:
- ivo://CDS.VizieR/J/A+A/551/A31
- Title:
- New spectral classification for hot subdwarfs
- Short Name:
- J/A+A/551/A31
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An MK (Morgan-Keenan)-like system of spectral classification for hot subdwarfs is presented. We find that a three-dimensional spectral type, consisting of a 'spectral' class, a 'luminosity' class, and a 'helium' class, is necessary to classify the sdO (subdwarf O) and sdB (subdwarf B) stars. In addition, the helium-strong stars appear to form two parallel spectral sequences: one showing strong lines of CII, CIII, or CIV, and the other with these same lines weak or absent. We also give a preliminary calibration of the new spectral types in terms of effective temperature, surface gravity, and surface helium-to-hydrogen abundance ratio, and show the relation between the new spectral types and the natural groups defined by the PG (Palomar-Green) survey.
