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Register a new userSpectral Energy Distributions of Be and Other Massive Stars
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We present spectrophotometric data from 0.4 to 4.2 microns for bright, northern sky, Be stars and several other types of massive stars. Our goal is to use these data with ongoing, high angular resolution, interferometric observations to model the density structure and sky orientation of the gas surrounding these stars. We also present a montage of the H-alpha and near-infrared emission lines that form in Be star disks. We find that a simplified measurement of the IR excess flux appears to be correlated with the strength of emission lines from high level transitions of hydrogen. This suggests that the near-IR continuum and upper level line fluxes both form in the inner part of the disk, close to the star.
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Context: The identification of long-gamma-ray-bursts (LGRBs) is still uncertain, although the collapsar engine of fast-rotating massive stars is gaining a strong consensus. Aims: We propose that low-metallicity Be and Oe stars, which are massive fast rotators, as potential LGRBs progenitors. Methods: We checked this hypothesis by 1) testing the global specific angular momentum of Oe/Be stars in the ZAMS with the SMC metallicity, 2) comparing the ZAMS ($Omega/Omega_{rm c},M/M_{odot}$) parameters of these stars with the area predicted theoretically for progenitors with metallicity $Z=0.002$, and 3) calculating the expected rate of LGRBs/year/galaxy and comparing them with the observed ones. To this end, we determined the ZAMS linear and angular rotational velocities for SMC Be and Oe stars using the observed vsini parameters, corrected from the underestimation induced by the gravitational darkening effect. Results: The angular velocities of SMC Oe/Be stars are on average $<Omega/Omega_{rm c}>=0.95$ in the ZAMS. These velocities are in the area theoretically predicted for the LGRBs progenitors. We estimated the yearly rate per galaxy of LGRBs and the number of LGRBs produced in the local Universe up to z=0.2. We have considered that the mass range of LGRB progenitors corresponds to stars hotter than spectral types B0-B1 and used individual beaming angles from 5 to 15degr. We thus obtain $R^{rm pred}_{rm LGRB}sim10^{-7}$ to $sim10^{-6}$ LGRBs/year/galaxy, which represents on average 2 to 14 LGRB predicted events in the local Universe during the past 11 years. The predicted rates could widely surpass the observed ones [(0.2-3)$times10^{-7}$ LGRBs/year/galaxy; 8 LGRBs observed in the local Universe during the last 11 years] if the stellar counts were made from the spectral type B1-B2, in accordance with the expected apparent spectral types of the appropriate massive fast rotators. Conclusion: We conclude that the massive Be/Oe stars with SMC metallicity could be LGRBs progenitors. Nevertheless, other SMC O/B stars without emission lines, which have high enough specific angular momentum, can enhance the predicted $R_{rm LGRB}$ rate.
Stars form in clustered environments, but how they form when the available resources are shared is still not well understood. A related question is whether the IMF is in fact universal across galactic environments, a galactic initial mass function (IGIMF), or whether it is an average of local IMFs. One of the long-standing problems in resolving this question and in the study of young clusters is observational: the emission from multiple sources is frequently seen as blended because at different wavelengths or with different telescopes the beam sizes are different. The confusion hinders our ability to fully characterize clustered star formation. Here we present a new method that uses a genetic algorithm and Bayesian inference to fit the blended SEDs and images of individual YSOs in confused clusters. We apply this method to the infrared photometry of a sample comprising 70 Spitzer-selected, low-mass ($M_{rm{cl}}<100~rm{M}_{odot}$) young clusters in the galactic plane, and use the derived physical parameters to investigate the distributions of masses and evolutionary stages of clustered YSOs, and the implications of those distributions for studies of the IMF and the different models of star formation. We find that for low-mass clusters composed of class I and class II YSOs, there exists a non-trivial relationship between the total stellar mass of the cluster ($M_{rm{cl}}$) and the mass of its most massive member ($m_{rm{max}}$). The properties of the derived correlation are most compatible with the random sampling of a Kroupa IMF, with a fundamental high-mass limit of $150~rm{M}_{odot}$. Our results are also compatible with SPH models that predict a dynamical termination of the accretion in protostars, with massive stars undergoing this stopping at later times in their evolution.
We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the first quiescent UV detections of these objects. All are detected at a much higher level than expected from their companion stars alone and are significant detections of the accretion flow. Three of the four UV excesses can be characterized by a black body of temperature 5000-13,000K, hotter than expected for the quiescent outer disk. A good fit could not be found for MM Vel. The source of the black-body-like emission is most likely a heated region of the inner disk. Contrary to initial indications from spectroscopy there does not appear to be a systematic difference in the UV luminosity or spectral shape between black holes and neutron star systems. However combining our new data with earlier spectroscopy and published X-ray luminosities there is a significant difference in the X-ray to UV flux ratios with the neutron stars exhibiting Lx/Luv about 10x higher than the black hole systems. Since both bandpasses are expected to be dominated by accretion light this suggests the difference in X-ray luminosities cannot simply reflect differences in quiescent accretion rates and so is a more robust discriminator between the black hole and neutron star populations than the comparison of X-ray luminosities alone.
(abridged) Spectral energy distributions (SEDs) were constructed for a sample of 477 classical cepheids (CCs). The SEDs were fitted with a dust radiative transfer code. Four stars showed a large mid- or far-infrared excess and the fitting then included a dust component. These comprise the well-known case of RS Pup, and three stars that are (likely) Type-II cepheids (T2Cs), AU Peg, QQ Per, and FQ Lac. The remainder of the sample was fitted with a stellar photosphere to derive the best-fitting luminosity and effective temperature. Distance and reddening were taken from the literature. The stars were plotted in a Hertzsprung-Russell diagram and compared to evolutionary tracks for cepheids and theoretical instability strips. For the large majority of stars, the position in the HRD is consistent with the instability strip for a CC or T2C. About 5% of the stars are outliers in the sense that they are much hotter or cooler than expected. A comparison to effective temperatures derived from spectroscopy suggests in some cases that the photometrically derived temperature is not correct and that this is likely linked to an incorrectly adopted reddening. In this work the presence of a small NIR excess, as has been proposed in the literature for a few well-known cepheids, is investigated. Firstly, this was done by using a sample of about a dozen stars for which a mid-infrared spectrum is available. Secondly, the SEDs of all stars were fitted with a dust model to see if a statistically significant better fit could be obtained. The results were compared to recent work. Eight new candidates for exhibiting a NIR excess are proposed, solely based on the photometric SEDs. Obtaining mid-infrared spectra would be needed to confirm this excess. Finally, period-bolometric luminosity and period-radius relations are presented for samples of over 370 fundamental-mode CCs.
According to recent theoretical studies, the progenitors of Long Gamma Ray Bursts should be very fast rotating stars, massive enough but not so for collapsing into a black hole. In addition, recent observations seem to show that stars of about 20 solar masses could be at the origin of LGRBs. At low metallicity B-type stars rotate faster than at higher metallicity. We found with the ESO-WFI an occurrence of Be/Oe stars, that are quasi critical rotators, 3 to 5 times larger in the SMC than in the Galaxy. According to our results, and using observational clues on the SMC WR stars, as well as the theoretical predictions of the characteristics must have the LGRB progenitors, we have identified the low metallicity massive Be/Oe stars as potential LGRB progenitors. To support this identification, the expected rates and the numbers of LGRB were then calculated and compared to the observed ones: 3 to 6 LGRBs were found in the local universe in 11 years while 8 were actually observed.
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