Stellar wind velocities and the radiation-driven winds theory for O stars

In this paper we have carried out an analysis of the predictions of the radiatively driven stellar winds theories on 63 stars belonging to clusters or associations. The spectral types in our sample range from O3 to B0 and all classes of luminosity are considered. The study has been carried out starting from the relationship between the stellar-wind velocity (v _edge) obtained from the resonance doublet ofCIV for stars observed with the IUE, and the escape velocity. The stellar masses have been obtained from the evolutionary tracks of Maeder and Meynet. Results from recent NLTE analyses with blanketing of lines and winds have been used for the effective temperature.Based on data from the International Ultraviolet Explorer, de-archived from the Villafranca Data Archive of the European Space Agency.     

A new numerical method for solving radiation driven winds from hot stars

We present a general method for solving the non‐linear differential equation of monotonically increasing steady‐state radiation driven winds. We graphically identify all the singular points before transforming the momentum equation to a system of differential equations with all the gradients explicitly given. This permits a topological classification of all singular points and to calculate the maximum and minimum mass‐loss of the wind. We use our method to analyse for the first time the topology of the non‐rotating frozen‐in ionisation m‐CAK wind, with the inclusion of the finite disk correction factor, and find up to 4 singular points, three of the x‐type and one attractor‐type. The only singular point (and solution passing through) that satisfies the boundary condition at the stellar surface is the standard m‐CAK singular point. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stellar models for very low-mass main-sequence stars: the role of model atmospheres

We present very low-mass stellar models as computed using non-grey model atmospheres for selected assumptions about the stellar metallicities. The role of atmospheres is discussed and the models are compared with models based on the Eddington approximation, and with similar models that have appeared in the recent literature. Theoretical predictions concerning both the HR diagram location and the mass–luminosity relation are presented and discussed in terms of expectations in selected photometric bands. Comparison with available observational data concerning both galactic globular clusters and dwarfs in the solar neighbourhood reveals a satisfactory agreement together with the existence of some residual mismatches.     

Pregalactic-primordial low-mass stars

The Main-Sequence positions as well as the evolutionary behavior of Population III stars up to an evolution age of 2×10¹⁰ yr, taking this time as the age of the Universe, have been investigated in the mass range 0.2 and 0.8M . While Population III stars with masses greater than 0.3M develop a radiative core during the approach to the Main Sequence, stars with masses smaller than 0.3M reach the Main Sequence as a wholly convective stars. Population III stars with masses greater than 0.5M show a brightening of at most 2.2 in bolometric magnitude when the evolution is terminated as compared to the value which corresponds to zero-age Main Sequence. The positions of stars with masses smaller than 0.5M remain almost the same in the H-R diagram.If Population III stars have formed over a range of redshifts, 6相似文献   

Monitoring the winds of hot stars with HST/GHRS

We present observations obtained with the Goddard High Resolution Spectrograph (GHRS) of three O-type stars: per (O7 III), Cep (O6 Iaf) and 10 Lac (O9 V). These observations show evidence of instabilities in the winds of all three stars, although not necessarily those expected from current models of structured winds.Based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA for NASA under contract NAS5-26555     

Pre-Main-Sequence evolution of rotating low-mass stars

The evolutionary behaviour of rotating low-mass stars in the mass range 0.2 and 0.9M has been investigated during the pre-Main-Sequence phase. The angular momentum is conserved locally in radiative regions and totally in convective regions, according to a predetermined angular velocity distribution depending on the structure of the star. As the stars contract toward the zero-age Main Sequence, they spin up under the assumption that the angular momentum is conserved during the evolution of the stars. When the stars have differential rotations, their inner regions rotate faster than the outer regions. The effective temperatures and luminosities of rotating low-mass stars are obtained lower than those of non-rotating stars. They have lower central temperature and density values compared to those of non-rotating stars.     

Gamma rays from colliding winds of massive stars

Colliding winds of massive binaries have long been considered as potential sites of non-thermal high-energy photon production. This is motivated by the detection of non-thermal spectra in the radio band, as well as by correlation studies of yet unidentified EGRET γ-ray sources with source populations appearing in star formation regions. This work re-considers the basic radiative processes and its properties that lead to high energy photon production in long-period massive star systems. We show that Klein–Nishina effects as well as the anisotropic nature of the inverse Compton scattering, the dominating leptonic emission process, likely yield spectral and variability signatures in the γ-ray domain at or above the sensitivity of current or upcoming gamma ray instruments like GLAST-LAT. In addition to all relevant radiative losses, we include propagation (such as convection in the stellar wind) as well as photon absorption effects, which a priori can not be neglected. The calculations are applied to WR 140 and WR 147, and predictions for their detectability in the γ-ray regime are provided. Physically similar specimen of their kind like WR 146, WR 137, WR 138, WR 112 and WR 125 may be regarded as candidate sources at GeV energies for near-future γ-ray experiments. Finally, we discuss several aspects relevant for eventually identifying this source class as a γ-ray emitting population. Thereby we utilize our findings on the expected radiative behavior of typical colliding wind binaries in the γ-ray regime as well as its expected spatial distribution on the γ-ray sky.     

High-resolution spectra of very low-mass stars

We present the results of high-resolution (1–0.4 Å) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I  −  K  > 3.5 ( M _bol ≳ 12.8) and those of more massive M dwarfs ( M _bol ≈ 7–10). A spectral atlas at high (0.4-Å) resolution for M8–M9+ stars is provided, and the equivalent widths of Cs  I , Rb  I and Hα lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of 'inactive, rapid rotators'. Such objects seem to be common at and below the hydrogen-burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism that heats the chromosphere, or the mechanism that generates magnetic fields, is greatly suppressed.     

Ion‐ion charge exchange and X‐rays from the winds of hot stars

Charge exchange occurs between charged ions with enough energy to overcome Coulomb repulsion, a condition satisfied for collisions at velocities like those of the winds driven from hot stars by radiation pressure. X‐ray line ratios in some hot stars are inconsistent with those expected from thermal plasmas excited by electron impact. Ion‐ion interactions including charge exchange might be responsible instead if high‐velocity collisions between ions are enabled by the presence of a magnetic field in the wind, suggesting a possible alternative mechanism to the widely accepted instability‐driven shock model. The nature of a plasma in charge‐exchange equilibrium is yet to be determined (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Carbon monoxide in low-mass dwarf stars

We compare high-resolution infrared observations of the CO 2–0 bands in the 2.297–2.310 μm region of M dwarfs and one L dwarf with theoretical expectations. We find a good match between the observational and synthetic spectra throughout the 2000–3500 K temperature regime investigated. None the less, for the 2500–3500 K temperature range, the temperatures that we derive from synthetic spectral fits are higher than expected from more empirical methods by several hundred kelvin. In order to reconcile our findings with the empirical temperature scale, it is necessary to invoke warming of the model atmosphere used to construct the synthetic spectra. We consider that the most likely reason for the back-warming is missing high-temperature opacity due to water vapour. We compare the water vapour opacity of the Partridge–Schwenke line list used for the model atmosphere with the output from a preliminary calculation by Barber & Tennyson. While the Partridge–Schwenke line list is a reasonable spectroscopic match for the new line list at 2000 K, by 4000 K it is missing around 25 per cent of the water vapour opacity. We thus consider that the offset between empirical and synthetic temperature scales is explained by the lack of hot water vapour used for computation of the synthetic spectra. For our coolest objects with temperatures below 2500 K, we find best fits when using synthetic spectra which include dust emission. Our spectra also allow us to constrain the rotational velocities of our sources, and these velocities are consistent with the broad trend of rotational velocities increasing from M to L.     

Nonthermal radio emission from hot star winds

Nonthermal radio emission has been observed from some of the most luminous hot star winds. It is understood to be synchrotron radiation of the relativistic electrons in the winds. To understand how the electrons are accelerated to such high energies and to correctly explain the observed radio flux and spectra require an exhaustive investigation of all the relevant physical processes involved and possibly point to a complex wind structure. In this paper we discuss the logical path toward a comprehensive model of the nonthermal radio emission from hot star winds. Based on the available observational data and fundamental theoretical considerations, we found that the only physically viable and self-consistent scenario is:the nonthermal radio emission is synchrotron radiation of relativistic electrons the electrons are accelerated by shocks via the first-order Fermi mechanism the acceleration has to be in situ in the radio emitting region the shocks formed at the base of the winds have to propagate to beyond the radio photosphere).     

Stellar winds and globules in Hii regions

The interaction of a plane-parallel hypersonic stellar wind with a globule in an Hii region is considered in two approximations. In both approximations, the ionization front on the globule remains strong-D type, and a flow pattern containing two oppositely facing shock waves results. In the first approximation, the structure of the shocked region is calculated assuming that globule gas and stellar wind gas mix well and move at the same velocity. However, this assumption results in a very thick shocked layer and the assumption of good mixing is consequently not well justified. This approximation provides an upper limit on the gas velocities expected in the shocked gas which originated at the globule. In the second approximation, the stellar wind merely applies pressure to balance the momentum flux in the globule gas. The structure of the shocked region is calculated on the assumption that a tangential discontinuity exists between shocked stellar wind and shocked glubule gas. Structures may be produced having velocities ～10 km s^?1 and emission measures ～10³ cm^?6 pc with reasonable stellar luminosities and mass loss rates.     

Infrared photometry of low-mass stars in Praesepe

We present follow-up infrared photometry for a sample of low-mass and very low-mass stars in the Praesepe open cluster. Our sample is selected from two sources: (i) 90 stars selected from the Hambly et al. photometric and proper-motion survey of Praesepe; (ii) 17 stars selected from the CCD imaging survey presented by Pinfield et al. We investigate cluster membership using infrared colour–magnitude and colour–colour diagrams. We find 81 likely and two possible members in the Hambly et al. sample, in line with predictions. Contamination in the Pinfield et al. sample is higher, and we find nine probable cluster members. We investigate the non-grey models of Baraffe et al., which are found to be in good agreement with the data. Multiplicity in Praesepe is also examined, and we find the multiple star fraction to be 0.51 from analysis of the I , I − K diagram. We investigate individual object masses, and find that the faintest candidate cluster members have masses close to the substellar limit.