It is now well established that stellar winds of hot stars are fragmentary and that the X-ray emission from stellar winds has a strong contribution from shocks in winds. Chandra high spectral resolution observations of line profiles of O and B stars have shown numerous properties that had not been expected. Here we suggest explanations by considering the X-rays as arising from bow shocks that occur where the stellar wind impacts on spherical clumps in the winds. We use an accurate and stable numerical hydrodynamical code to obtain steady-state physical conditions for the temperature and density structure in a bow shock. We use these solutions plus analytic approximations to interpret some major X-ray features: the simple power-law distribution of the observed emission measure derived from many hot star X-ray spectra and the wide range of ionization stages that appear to be present in X-ray sources throughout the winds. Also associated with the adiabatic cooling of the gas around a clump is a significant transverse velocity for the hot plasma flow around the clumps, and this can help to understand anomalies associated with observed line widths, and the differences in widths seen in stars with high and low mass-loss rates. The differences between bow shocks and the planar shocks that are often used for hot stars are discussed. We introduce an ``on the shock'' (OTSh) approximation that is useful for interpreting the X-rays and the consequences of clumps in hot star winds and elsewhere in astronomy.
Clumping in the winds of massive stars may significantly reduce empirical mass-loss rates, and which in turn may have a large impact on our understanding of massive star evolution. Here, we investigate wind-clumping through the linear polarization induced by light scattering off the clumps. Through the use of an analytic wind clumping model, we predict the time evolution of the linear polarimetry over a large parameter space. We concentrate on the Luminous Blue Variables, which display the greatest amount of polarimetric variability and for which we recently conducted a spectropolarimetric sur…
Recent results strongly challenge the canonical picture of massive star winds: various evidence indicates that currently accepted mass-loss rates, Mdot, may need to be revised downwards significantly. This is because the most commonly used mass-loss diagnostics are affected by ``clumping'' (small-scale density inhomogeneities), influencing our interpretation of observed spectra and fluxes. Such downward revisions would have dramatic consequences for the evolution of, and feedback from, massive stars, and thus robust determinations of the clumping properties and mass-loss rates are urgently nee…
We present high angular resolution interferometric observations of the 3 and 1.3mm continuum emission, and HCO+(1-0) and SiO(2-1)v=0 lines, obtained with the Owens Valley Radio Observatory millimeter-wave array, toward two hot cores (HCs) associated with two well known ultracompact (UC) HII regions: G29.96-0.02 and G31.41+0.31. These HCs are believed to host young forming massive stars which have been suggested to be surrounded by massive rotating accretion disks. The aim of these new observations is to study the structure and kinematics of the molecular clumps surrounding the HCs and nearby U…
We report the detection and monitoring of transient substructures in the radiation-driven winds of five massive, hot stars in different evolutionary stages. Clumping in the winds of these stars shows up as variable, narrow subpeaks superposed on their wide, wind-broadened (optical) emission lines. Similar patterns of emission-line profile variations are detected in the Of stars zeta Puppis and HD93129A, in the more evolved hydrogen-rich, luminous, Of-like WN stars HD93131 and HD93162, and in the more mass-depleted WC star in gamma2 Velorum. These observations strongly suggest that stochastic w…
This paper reports the diffuse X-ray features around the Galactic center observed with Chandra. We confirm the ASCA and Ginga discoveries of the large-scale thin-thermal plasma with strong lines in the Galactic center region. In addition, many small clumps of emission lines from neutral (6.4 keV line) to He-like (6.7 keV line) irons are discovered. The 6.4 keV line clumps would be reflection nebulae, while those of the 6.7 keV line are likely SNRs. We also find emission lines of intermediate energy between 6.5-6.7 keV, which are attributable to young SNRs in non equilibrium ionization. Non-the…
We present the results of a targeted 3-mm spectral line survey towards the eighty-three 6.67 GHz methanol maser selected star forming clumps observed by Purcell et al. 2006. In addition to the previously reported measurements of HCO+ (1 - 0), H13CO+ (1 - 0), and CH3CN (5 - 4) & (6 -5), we used the Mopra antenna to detect emission lines of N2H+ (1 - 0), HCN (1 - 0) and HNC (1 - 0) towards 82/83 clumps (99 per cent), and CH3OH (2 - 1) towards 78/83 clumps (94 per cent). The molecular line data have been used to derive virial and LTE masses, rotational temperatures and chemical abundances in the …
We present a study of hydrodynamic drag forces in smoothed particle simulations. In particular, the deceleration of a resolution-limited cold clump of gas moving through a hot medium is examined. It is found that the drag for subsonic velocities exceeds that predicted by simple physical approximations. This is shown to be a result of the hydrodynamical method which encourages the accretion of particles from the hot medium onto a shell around the cold clump, effectively increasing the radius of the clump. For sonic and supersonic velocities, the drag is shown to be dependent on the effective cr…
We present XMM-Newton observations of gamma^2 Velorum (WR 11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Ray binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a…
We present a submm study of the massive hot core G327.3-0.6 that constrains its physical parameters and environment. The APEX telescope was used to image CO and N2H+ emission, to observe lines from other molecules toward a hot and a cold molecular core, and to measure the continuum flux density of the hot core. In the C18O J=3-2 line, two clumps were found, one associated with the HII region G327.3-0.5 and the other associated with the hot core. An additional cold clump is found 30 arcsec (0.4 pc) northeast of the hot core in bright N2H+ emission. From the the continuum data, we calculate a ma…
We compare the hot star wind models calculated assuming older solar abundance determination with models calculated using the recently published values derived from 3D hydrodynamical model atmospheres. We show that the use of new abundances with lower metallicity improves the agreement between wind observation and theory in several aspects: (1) The predicted wind mass-loss rates are lower by a factor of 0.76. This leads to a better agreement with mass-loss rate determinations derived from observations with account of clumping. (2) As a result of the lowering of mass-loss rates, there is a bette…
We have compared Monte Carlo photoionization models of H II regions with a uniform density distribution with models with the same central stars and chemical compositions but with 3-D hierarchical clumps. We compare the abundances of He, N, O, Ne, and S obtained from emission line strengths and [O III] and [N II] temperatures to those in our models. We consider stellar temperatures in the range 37.5 -- 45kK and ionizing luminosities from 10^{48} to 10^{51} photons/s. Clumped models have different ionic abundances than uniform. For hot stars, He^0/He^+ is 2 -- 3%, much larger than with uniform m…
Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively to be successfully included in broader astrophysical applications. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on winds from OB-stars, with some excursions to the Luminous Blue Variables, Wolf- Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After reca- pitulating the 1-D, stationary standard model of line-driven wind, extensions accounting for rotation and magnetic fields are discusse…
We present the results of an investigation of the effects of Far Ultraviolet (FUV) radiation from hot early type OB stars on clumps in star-forming molecular clouds. Clumps in Photodissociation regions (PDRs) undergo external heating which, if rapid, creates strong photoevaporative mass flows off the clump surfaces, and drives shocks into the clumps, compressing them to high densities. The clumps lose mass on relatively short timescales. The evolution of an individual clump is found to be sensitive to its initial colunm density, the temperature of the heated surface and the ratio of the ``turn…
The topic of wind-clumping has been the subject of much activity in recent years, due to the impact that it can have on derived mass-loss rates. Here we present an alternative method of investigating wind-clumping, that of polarimetry. We present simulations of the polarization produced by a clumpy wind, and argue that the observations may be reproduced just by statistical deviations from spherical symmetry when the outflow is only slightly fragmented. Here, the polarization scales with $\dot{M}$, which is consistent with observations of LBVs, WRs and O supergiants. Finally, we find clumping f…
The hot star wind momentum problem η= mdot*vinfty/(L/c) >> 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate mdot, and hence the momentum supply, required to generate a specified radio emission measure \varepsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in oppositi…
The nature of the internal thermal X-ray emission seen in ``thermal composite" supernova remnants is still uncertain. Chandra observation of the 3C391 shows a southeast-northwest elongated morphology and unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas comparatively favor the cloudlet evaporation model as a main mechanism for the ``thermal composite" X-ray appearance, though radiative rim and thermal conduction ma…
Clumping in hot star winds can significantly affect estimates of mass-loss rates, the inferred evolution of the star and the environmental impact of the wind. A hydrodynamical simulation of a colliding winds binary (CWB) with clumpy winds reveals that the clumps are rapidly destroyed after passing through the confining shocks of the wind-wind collision region (WCR) for reasonable parameters of the clumps if the flow in the WCR is adiabatic. Despite large density and temperature fluctuations in the post-shock gas, the overall effect of the interaction is to smooth the existing structure in the …
Context: The mass-loss rate is a key parameter of massive stars. Adequate stellar atmosphere models are required for spectral analyses and mass-loss determinations. Present models can only account for the inhomogeneity of stellar winds in the approximation of small-scale structures that are optically thin. This treatment of ``microclumping'' has led to reducing empirical mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be optically thick in spectral lines. We investigate how this ``macroclumping'' impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (P…
Archival X-ray spectra of the four prominent single, non-magnetic O stars Zeta Pup, Zeta Ori, Ksi Per and Zeta Oph, obtained in high resolution with Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles provide information about the shocked, hot gas which emits the X-radiation, and about the bulk of comparably cool stellar wind material which partly absorbs this radiation. In this paper, we synthesize X-ray line profiles with a model of a clumpy stellar wind. We find that the geometrical shape of the wind inhomogeneities is important: better agreement with the observat…
The massive galaxies in the young universe, ten billion years ago, formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids (Genzel et al. 2006, 2008). Cosmological simulations and clustering theory are used to explore how these galaxies acquired their gas. Here we report that they are stream-fed galaxies, formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark …
We reanalyze the ROSAT PSPC X-ray spectro-photometric observations of HCG 16. Its X-ray morphology resembles its morphology at 20 cm, seen by the NVSS. We detect diffuse emission in 8 regions filling half of the 200/h50 kpc (8.7") radius circle around the optical center of the group: one encompassing galaxies a&b, two surrounding galaxies c&d, a clumpy region roughly 140/h50 kpc from the group galaxies, which may be gas ejected from one of the galaxies, plus regions associated with background galaxies, radio-sources or groups, and foreground stars. Excluding the background / foreground sources…
We present results from the first extensive study of convection zones in the envelopes of hot massive stars, which are caused by opacity peaks associated with iron and helium ionization. These convective regions can be located very close to the stellar surface. Recent observations of microturbulence in massive stars from the VLT-Flames survey are in good agreement with our predictions concerning the occurrence and the strength of sub-surface convection in hot stars. We argue further that convection close to the surface may trigger clumping at the base of the stellar wind of massive stars.
A study of 12 ultracompact HII regions was conducted to probe the physical conditions and kinematics in the inner envelopes of the molecular clumps harboring them. The APEX telescope was used to observe the sources in the CO (4-3) and 13CO (8-7) lines. Line intensities were modeled with the RATRAN radiative transfer code using power laws for the density and temperature to describe the physical structure of the clumps. All sources were detected in both lines. The optically thick CO (4-3) line shows predominantly blue skewed profiles reminiscent of infall. Line intensities can be reproduced well…
The mass-loss rates of hot, massive, luminous stars are considered a decisive parameter in shaping the evolutionary tracks of such stars and influencing the interstellar medium on galactic scales. The small-scale structures (clumps) omnipresent in such winds may reduce empirical estimates of mass-loss rates by an evolutionarily significant factor of >=3. So far, there has been no direct observational evidence that wind-clumping may persist at the same level in environments with a low ambient metallicity, where the wind-driving opacity is reduced. Here we report the results of time-resolved spe…
We present molecular line mapping of the Giant Molecular Cloud G1.6-0.025, which is located at the high longitude end of the Central Molecular Zone of our Galaxy. We assess the degree of star formation activity in that region using several tracers and find very little. We made a large scale, medium (2') resolution map in the J = 2-1 transition of SiO for which we find clumpy emission over a ~0.8 x 0.3 degree-sized region stretching along the Galactic plane. Toward selected positions we also took spectra in the easy to excite J_k=2_k-1_k quartet of CH3OH and the CS 2-1 line. Throughout the clou…
X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate from optically thin shock-heated plasma deep inside the wind and propagate outwards throughout absorbing cool material. Recent analyses of the line ratios from He-like ions in the X-ray spectra of O-stars highlighted problems with this general paradigm: the measured line ratios of highest ions are consistent with the location of the hottest X-ray emitting plasma very close to the base of the wind, perhaps indicating the presence of a corona, while measurements from lower ions conform with the wind-embedded shock model. …
The impact of FUSE upon the fundamental parameters of OB stars and Wolf-Rayet stars is reviewed. The stellar wind signatures available in the far-UV provide us with important additional diagnostics of effective temperature. Together with improved non-LTE stellar atmosphere models allowing for line blanketing and stellar winds, this has led to a downward revision in the spectral type-temperature calibration for O stars versus Vacca et al. (1996) In addition, the Lyman continuum ionizing fluxes from O dwarfs are compared with previous calibrations of Panagia (1973) and Vacca et al. We also discu…
We present the results of Monte Carlo mass-loss predictions for massive stars covering a wide range of stellar parameters. We critically test our predictions against a range of observed mass-loss rates -- in light of the recent discussions on wind clumping. We also present a model to compute the clumping-induced polarimetric variability of hot stars and we compare this with observations of Luminous Blue Variables, for which polarimetric variability is larger than for O and Wolf-Rayet stars. Luminous Blue Variables comprise an ideal testbed for studies of wind clumping and wind geometry, as wel…
