Charge exchange in massive star‐forming regions

As a result of feedback from massive stars, via their intense winds and/or supernova explosions, massive star‐forming regions are entirely filled with hot, X‐ray emitting plasmas, which escape into the ambient ISM. As shown recently by Townsley et al. for several “extreme” cases (Carina, M17, NGC 3576, NGC 3603, 30 Dor), by way of large Chandra ACIS mosaics, extra, non‐thermal emission lines are present on top of the standard lines emitted by hot plasmas. Some of them are very close to lines characteristic of charge‐exchange reactions between the hot plasma and the cold surrounding material, suggesting that this mechanism operates on large spatial scales (several 10 pc) in star‐forming regions in general. The connection with starburst galaxies is briefly mentioned, and it is pointed out that supernovae interacting with molecular clouds may also provide a good environment to look for charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐ray spectroscopy of early‐type stars: The present and the future

XMM‐Newton and Chandra have boosted our knowledge about the X‐ray emission of early‐type stars (spectral types OB and Wolf‐Rayet). However, there are still a number of open questions that need to be addressed in order to fully understand the X‐ray spectra of these objects. Many of these issues require high‐resolution spectroscopy or monitoring of a sample of massive stars. Given the moderate X‐ray brightness of these targets, rather long exposure times are needed to achieve these goals. In this contribution, we review our current knowledge in this field and present some hot topics that could ideally be addressed with XMM‐Newton over the next decade. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Masses and luminosities of O‐ and B‐type stars and red supergiants

Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O‐ and early B‐type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi‐colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O‐ and early B‐type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground‐based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ≈20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Charge transfer reactions at interfaces between neutral gas and plasma: Dynamical effects and X‐ray emission

Charge‐transfer is the main process linking neutrals and charged particles in the interaction regions of neutral (or partly ionized) gas with a plasma. In this paper we illustrate the importance of charge‐transfer with respect to the dynamics and the structure of neutral gas‐plasma interfaces. We consider the following phenomena: (1) the heliospheric interface ‐ region where the solar wind plasma interacts with the partly‐ionized local interstellar medium (LISM) and (2) neutral interstellar clouds embedded in a hot, tenuous plasma such as the million degree gas that fills the so‐called “Local Bubble”. In (1), we discuss several effects in the outer heliosphere caused by charge exchange of interstellar neutral atoms and plasma protons. In (2) we describe the role of charge exchange in the formation of a transition region between the cloud and the surrounding plasma based on a two‐component model of the cloud‐plasma interaction. In the model the cloud consists of relatively cold and dense atomic hydrogen gas, surrounded by hot, low density, fully ionized plasma. We discuss the structure of the cloud‐plasma interface and the effect of charge exchange on the lifetime of interstellar clouds. Charge transfer between neutral atoms and minor ions in the plasma produces X‐ray emission. Assuming standard abundances of minor ions in the hot gas surrounding the cold interstellar cloud, we estimate the X‐ray emissivity consecutive to the charge transfer reactions. Our model shows that the charge‐transfer X‐ray emission from the neutral cloud‐plasma interface may be comparable to the diffuse thermal X‐ray emission from the million degree gas cavity itself (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A search for magnetic fields in cool sdB stars

Hot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐shooting Herbig Ae/Be stars: Accretion probed by near‐infrared He I emission

The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinematics of 11 Bidelman's weak‐lined stars

Radial velocities obtained with the CORAVEL‐type spectrometer are presented for 11 Bidelman's weak‐lined stars, classified from moderate dispersion objective prism spectra. Standard errors of the measurements are smaller than 1 km/s. For five of these stars, measured in the Vilnius photometric system, only small metal deficiency is established. Kinematically, most of the stars belong to the thin disk and only two stars, HD 71185 and BD +75° 641, are consistent with membership of the thick disk population.     

Pulsations and planets: The asteroseismology‐extrasolar‐planet connection

The disciplines of asteroseismology and extrasolar planet science overlap methodically in the branch of high‐precision photometric time series observations. Light curves are, amongst others, useful to measure intrinsic stellar variability due to oscillations, as well as to discover and characterize those extrasolar planets that transit in front of their host stars, periodically causing shallow dips in the observed brightness. Both fields ultimately derive fundamental parameters of stellar and planetary objects, allowing to study for example the physics of various classes of pulsating stars, or the variety of planetary systems, in the overall context of stellar and planetary system formation and evolution. Both methods typically also require extensive spectroscopic follow‐up to fully explore the dynamic characteristics of the processes under investigation. In particularly interesting cases, a combination of observed pulsations and signatures of a planet allows to characterize a system's components to a very high degree of completeness by combining complementary information. The planning of the relevant space missions has consequently converged with respect to science cases, where at the outset there was primarily a coincidence in instrumentation and techniques. Whether space‐ or ground‐based, a specific type of stellar pulsations can themselves be used in an innovative way to search for extrasolar planets. Results from this additional method at the interface of stellar pulsation studies and exoplanet hunts in a beyond‐mainstream area are presented (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variability and evolution in various classes of post‐AGB stars

We aim to compare properties of early‐type post‐asymptotic giant‐branch (post‐AGB) stars, including normal first‐time B‐type post‐AGB stars, and extreme helium stars (EHes). Hipparcos photometry for 12 post‐AGB stars and 7 EHe stars has been analyzed; 5 post‐AGB stars are clearly variable. The Hipparcos data are not sufficiently sensitive to detect variability in any of the EHes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic survey of emission line B‐type stars with FORS 1 at the VLT

We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission‐line B‐type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B‐type stars and He‐strong/He‐weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi‐mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈B_z〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈B_z〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B‐type stars out of the observed fourteen cluster members (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Proper motion and X‐ray selected search for new members of the young TW Hya association

We have searched for new members of the TWHya association (TWA) among unidentified ROSAT X‐ray sources by identifying them in proper motion catalogues and selecting those that would be consistent with kinematical membership to the TWA. Spectroscopic follow‐up observations of 19 member candidates revealed the detection of moderate lithium absorption lines for the following three stars: GSC 7206 845, TYC 7216‐55, and TYC 7247‐12. The isochronal ages of the latter TYC stars are estimated to be ∼20 Myr while the other one has ∼100 Myr age based on a kinematic distance estimate that assumes TWA membership. However, the moderately Li‐rich stars are not likely to be new pre‐main sequence members of TWA partly because of the discrepant radial velocities. Infrared follow‐up imaging in the H‐band for the 3 stars shows companion candidates near two of them.While one system (TYC 7216‐55) is probably a near‐equal‐magnitude stellar binary, our follow‐up H‐band spectrum of the faint companion candidate near GSC 7206 845 shows that it is instead a background K‐type star rather than a companion. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

>“Resolving” neutron stars physics and geometry

The unprecedented harvest of X‐ray photons detected from dozens of isolated neutron stars has made it possible to glimpse at their emission mechanisms as well as at their emission geometry. Rotating hot spot(s), superimposed to the global thermal emission from the neutron star surface, are seen from several objects, allowing to probe the stars' external heating sources. Non‐thermal emission is also seen to vary as the stars rotate. Moreover, absorption features have been detected in the spectra of several objects, allowing to probe (tentatively) the stars' magnetic fields. Spectacular tails, trailing the stars' supersonic motion, trace the boundaries of the relativist winds streaming from the star's magnetosphere. Apart from classical radio pulsar and certified radio‐quiet neutron stars, XMM‐Newton has devoted significant observation time to the enigmatic central compact objects, presumably isolated neutron stars shining at the center of their supernova remnants. Far from showing a unifying behaviour, XMM‐Newton data have unveiled a surprising diversity. Understanding the reason(s) behind such diversity is the challenge for the next decade of X‐ray observations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The kinematic characteristics of magnetic O‐type stars

Although magnetic fields have been discovered in ten massive O‐type stars during the last years, the origin of their magnetic fields remains unknown. Among the magnetic O‐type stars, two stars, HD 36879 and HD 57682, were identified as candidate runaway stars in the past, and θ¹ Ori C was reported to move rapidly away from its host cluster. We search for an explanation for the occurrence of magnetic fields in O‐type stars by examining the assumption of their runaway status. We use the currently best available astrometric, spectroscopic, and photometric data to calculate the kinematical status of seven magnetic O‐type stars with previously unknown space velocities. The results of the calculations of space velocities suggest that five out of the seven magnetic O‐type stars can be considered as candidate runaway stars. Only two stars, HD 155806 and HD 164794, with the lowest space velocities, are likely members of Sco OB4 and NGC 6530, respectively. However, the non‐thermal radio emitter HD 164794 is a binary system with colliding winds, for which the detected magnetic field has probably a different origin in comparison to other magnetic O‐type stars (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variability of young stars: Determination of rotational periods of weak‐line T Tauri stars in the Cepheus‐Cassiopeia star‐forming region

We report on observation and determination of rotational periods of ten weak‐line T Tauri stars in the Cepheus‐Cassiopeia star‐forming region. Observations were carried out with the Cassegrain‐Teleskop‐Kamera (CTK) at University Observatory Jena between 2007 June and 2008 May. The periods obtained range between 0.49 d and 5.7 d, typical for weak‐line and post T Tauri stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solar system X‐rays from charge exchange processes

While X‐ray astronomy began in 1962 and has made fast progress since then in expanding our knowledge about where in the Universe X‐rays are generated by which processes, it took one generation before the importance of a fundamentally different process was recognized. This happened in our immediate neighborhood, when in 1996 comets were discovered as a new class of X‐ray sources, directing our attention to charge exchange reactions. Charge exchange is fundamentally different from other processes which lead to the generation of X‐rays, because the X‐rays are not produced by hot electrons, but by ions picking up electrons from cold gas. Thus it opens up a new window, making it possible to detect cool gas in X‐rays (like in comets), while all the other processes require extremely high temperatures or otherwise extreme conditions. After having been overlooked for a long time, the astrophysical importance of charge exchange for the generation of X‐rays is now receiving increased general attention. In our solar system, charge exchange induced X‐rays have now been established to originate in comets, in all the planets from Venus to Jupiter, and even in the heliosphere itself. In addition to that, evidence for this X‐ray emission mechanism has been found at various locations across the Universe. Here we summarize the current knowledge about solar system X‐rays resulting from charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tracing low‐energy cosmic‐rays by charge exchange X‐ray emission

Hadronic cosmic rays of energies below about 100 MeV nucleon^–1 are thought to be an important component of the Galactic ecosystem. However, since these particles cannot be detected near Earth due to the solar modulation effect, their composition and flux in the interstellar medium are very uncertain. Atomic interactions of low‐energy cosmic rays with interstellar gas can produce a characteristic nonthermal X‐ray emission comprising very broad lines from de‐excitations in fast ions following charge exchange. We suggest that broad lines at ∼0.57 and ∼0.65 keV could be detected from a dark molecular cloud in the local interstellar medium. These lines would be produced by fast oxygen ions of kinetic energies around 1 MeV nucleon^–1 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studying the formation of massive stars with VLT/X‐shooter

The birth process and (early) evolution of massive stars is still poorly understood. Massive stars are rare, their birthplaces are hidden from view and their formation timescale is short. So far, our physical knowledge of these young massive stars has been derived from near‐IR imaging and spectroscopy, revealing populations of young OB‐type stars, some still surrounded by a (remnant?) accretion disk, others apparently “normal” main sequence stars powering H II regions. The most important spectral features of OB‐type stars are, however, located in the UV and optical range. With VLT/X‐shooter it is possible to extend the spectral coverage of these young massive stars into the optical range, to better determine their photospheric properties, to study the onset of the stellar wind, and to characterize the physical structure of the circumstellar disk (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Update on modeling and data analysis of heliospheric solar wind charge exchange X‐ray emission

About 15 years ago, charge exchange (CX) X‐ray emission was discovered in comet observations, and was identified as the radiative decay of excited states of highly‐charge solar wind ions populated in collisions with neutral cometary material. This non‐thermal X‐ray emission mechanism is now generally acknowledged in planetary environments (e.g. Mars, Earth), as well as interstellar atoms sweeping through the heliosphere. In this paper I present the most recent improvements made in simulations of the heliospheric CX X‐ray emission. The model results are compared to X‐ray data from Suzaku, XMM‐Newton and Chandra spanning over a 10‐year period, and some conclusions are drawn on the heliospheric contribution to the diffuse soft X‐ray background. The solar system CX X‐ray sources can serve as prototypes in terms of modeling and diagnostics to more distant astrophysical objects where CX emission signatures are being discovered (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ROSAT HRI observations of P Cyg and surrounding area

We present a detailed analysis of deep ROSAT HRI observations of the luminous blue variable P Cyg and its surrounding radio nebula. The HRI image provides a point source at the position of P Cyg. However, we show that this emission can be attributed to the ultraviolet leak of the ROSAT HRI. The X‐ray flux upper limit derived from the HRI data of this star is discussed in the context of X‐ray emission from hot stars. Furthermore, we present a search for diffuse X‐ray emission possibly associated with the radio nebula surrounding P Cyg. We compare our results to model predictions and X‐ray fluxes observed for shells around other hot stars. Additionally, we detect 10 X‐ray sources in the field of view. All but one of these X‐ray emitters have stellar counter parts in the Palomar Sky Survey. We suggest that they are active late‐type stars possibly belonging to Cyg OB1.     

Chemical composition of a sample of bright solar‐metallicity stars

We present a detailed analysis of seven young stars observed with the spectrograph SOPHIE at the Observatoire de Haute‐Provence for which the chemical composition was incomplete or absent in the literature. For five stars, we derived the stellar parameters and chemical compositions using our automatic pipeline optimized for F, G, and K stars, while for the other two stars with high rotational velocity, we derived the stellar parameters by using other information (parallax), and performed a line‐by‐line analysis. Chromospheric emission‐line fluxes from Caii are obtained for all targets. The stellar parameters we derive are generally in good agreement with what is available in the literature. We provide a chemical analysis of two of the stars for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm implying a high lithium abundance. Its chemical pattern is not consistent with it being a solar sibling, as has been suggested. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)