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1.
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)  相似文献   

2.
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)  相似文献   

3.
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)  相似文献   

4.
Archival HST FOS and GHRS data sets have been used to collect ultraviolet evidence for large- and small-scale stellar wind structure in extragalactic Local Group OB stars (i.e. SMC, LMC including R136, M31, M33 and NGC 6822). By comparison with previous studies of Galactic OB stars, wind activity is principally diagnosed in individual spectrograms via the presence of 'narrow absorption components' and saturated 'black' absorption troughs in the resonance line doublets. Their characteristics broadly suggest that these stars share the same physical mechanisms for perturbing the winds as those that act in Galactic stars. Both of these spectral indicators are also used to provide reliable measures of wind terminal velocities. These velocities are directly compared with previously published Galactic values, without reliance on model profile fitting. Relative to Galactic OB stars, the most discrepant terminal velocities (and wind line profiles) result from main-sequence early O-type stars in the SMC.  相似文献   

5.
The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrow-band spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main-sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines.
Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1–2 Myr) objects, and dwarf and giant field stars.  相似文献   

6.
We report on the discovery of over 50 strong Hα emitting objects towards the large OB association Cyg OB2 and the H  ii region DR 15 on its southern periphery. This was achieved using the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r ',  i ' and H α photometry from IPHAS, and near-infrared J ,  H and K photometry from Two Micron All Sky Survey. The position of the objects in the ( J − H ) versus ( H − K ) diagram strongly suggests most of them are young. Many show Ca  ii infrared triplet emission indicating that they are in a pre-main-sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the H  ii region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable. The emission-line objects towards Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main-sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.  相似文献   

7.
We present high‐quality spectra of the majority of stars that have been classified as Oe and find that their published spectral types are generally too early, most likely due to infilling of He I lines. As a matter of fact, all stars classified as Oe actually fall inside the range O9–B0 with the important exception of HD 155806 (O7.5 III) and perhaps HD 39680 (difficult to classify, but likely O8.5V). Observations of a sample of objects with published spectral types in the O9–B0 range previously classified as peculiar or emission‐line stars fail to reveal any new Oe star with spectral type earlier than O9.5. Most objects classified as peculiar in “classical” literature show signs of binarity in our spectra, but no spectral anomalies. We conclude that there is likely a real decline in the fraction of Be stars for spectral types earlier than B0, not due to observational bias. The few Oe stars with spectral types earlier than O9.5 deserve detailed investigation in order to provide constraints on the physical reasons of the Be phenomenon. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
A detailed study was performed for a sample of low-mass pre-main-sequence (PMS) stars, previously identified as weak-line T Tauri stars, which are compared to members of the Tucanae and Horologium Associations. Aiming to verify if there is any pattern of abundances when comparing the young stars at different phases, we selected objects in the range from 1 to 100 Myr, which covers most of PMS evolution. High-resolution optical spectra were acquired at European Southern Observatory and Observatório do Pico dos Dias . The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of iron absorption lines. Chemical abundances were obtained via equivalent width calculations and spectral synthesis for 44 per cent of the sample, which shows metallicities within 0.5 dex solar. A classification was developed based on equivalent width of Li  i 6708 Å and Hα lines and spectral types of the studied stars. This classification allowed a separation of the sample into categories that correspond to different evolutive stages in the PMS. The position of these stars in the Hertzsprung–Russell diagram was also inspected in order to estimate their ages and masses. Among the studied objects, it was verified that our sample actually contains seven weak-line T Tauri stars, three are Classical T Tauri, 12 are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequence stars. An estimation of circumstellar luminosity was obtained using a disc model to reproduce the observed spectral energy distribution. Most of the stars show low levels of circumstellar emission, corresponding to less than 30 per cent of the total emission.  相似文献   

9.
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.  相似文献   

10.
Stellar magnetic fields govern key aspects of the evolution of a young star, from controlling accretion to regulating the angular momentum evolution of the system. Spectro‐polarimetric studies of T Tauri stars have revealed a surprising range of magnetic field topologies. Meanwhile multi‐wavelength campaigns have probed T Tauri star systems from stellar photosphere to inner disk, allowing us to study magnetospheric accretion in unprecedented detail. We review recent results and discuss their implications for understanding the evolution of young stars (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
X-rays from massive OB stars: thermal emission from radiative shocks   总被引：1，自引：0，他引：1  
Chandra grating spectra of a sample of 15 massive OB stars were analysed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by a relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete or fractional shells) eventually become 'cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the 'cold' clouds, resulting in blueshifted spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to ∼0.1–0.4 keV and (ii) the other with X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiation-driven instability shocks and in magnetically confined wind shocks, respectively.  相似文献   

12.
The picture of the young stellar groups in the Canis Major–Puppis–Vela (215°< l <275°) section of the Milky Way is studied and updated utilizing uvbyβ photometry of intrinsically luminous OB stars. We use all data from the literature to create a sample with 98 per cent completeness to 9.5 mag.
The very dense low reddened OB association CMa OB1 is confirmed at a distance of 0.99 (±0.05 s.e. ) kpc. Towards Puppis the brightest intrinsically luminous stars do not reveal Pup OB1 and Pup OB2. In the same direction, we separate two small groups, previously related to the association surrounding NGC 2439 at 3.5–4.5 kpc. The first one contains four highly reddened B-type supergiants situated in front of the cluster at 1.03 (±0.14 s.e.) kpc – much closer to the Sun than has been estimated before. The second one lies north-west from the cluster at 3.2 (±0.23 s.e.) kpc according to our estimate. In the direction to Vela, the bright OB stars are apparently embedded in a dust cloud and spread out between 0.3 and 2.5 kpc, forming clumps over this distance range.
In general, the prominent apparent young structures delineated by the brightest intrinsically luminous OB stars in the directions of Canis Major and Vela are some 20–25 per cent closer to the Sun than has previously been thought. This is in agreement with the Hipparcos results for the Galactic OB associations, and is highly likely to be caused by the overestimation of the spectroscopic distances used in the previous studies.  相似文献   

13.
As Be stars are restricted to luminosity classes III‐V, but early B‐type stars are believed to evolve into supergiants, it is to be expected that the Be phenomenon disappears at some point in the evolution of a moderately massive star, before it reaches the supergiant phase. As a first stage in an attempt to determine the physical reasons of this cessation, a search of the literature has provided a number of candidates to be Be stars with luminosity classes Ib or II. Spectroscopy has been obtained for candidates in a number of open clusters and associations, as well as several other bright stars in those clusters. Among the objects observed, HD 207329 is the best candidate to be a high‐luminosity Be star, as it appears like a fast‐rotating supergiant with double‐peaked emission lines. The lines of HD 229059, in Berkeley 87, also appear morphologically similar to those of Be stars, but there are reasons to suspect that this object is an interacting binary. At slightly lower luminosities, LS I +56°92 (B4 II) and HD 333452 (O9 II), also appear as intrinsically luminous Be stars. Two Be stars in NGC 6913, HD 229221 and HD 229239, appear to have rather higher intrinsic magnitudes than their spectral type (B0.2 III in both cases) would indicate, being as luminous as luminosity class II objects in the same cluster. HD 344863, in NGC 6823, is also a rather early Be star of moderately high luminosity. The search shows that, though high‐luminosity Be stars do exist, they are scarce and, perhaps surprisingly, tend to have early spectral types. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
Stellar photometry derived from the INT/WFC Photometric Hα Survey (IPHAS) of the Northern Galactic plane can be used to identify large, reliable samples of A0–A5 stars. For every A-type star, so identified, it is also possible to derive individual reddening and distance estimates, under the assumption that most selected objects are on or near the main sequence, at a mean absolute r ' magnitude of 1.5–1.6. This study presents the method for obtaining such samples and shows that the known reddenings and distances to the open clusters NGC 7510 and NGC 7790 are successfully recovered. A sample of over 1000 A-type stars is then obtained from IPHAS data in the magnitude range  13.5 < r ' < 20  from the region of sky including the massive northern OB association Cyg OB2. An analysis of these data reveals a concentration of ∼200 A stars over an area about a degree across, offset mainly to the south of the known 1–3 Myr old OB stars in Cyg OB2: their dereddened r ' magnitudes fall in the range 11.8–12.5. These are consistent with a ∼7 Myr old stellar population at distance modulus DM = 10.8, or with an age of ∼5 Myr at DM = 11.2. The number of A-type stars found in this clustering alone is consistent with a lower limit to the cluster mass of  ∼104 M⊙  .  相似文献   

15.
Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour–magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.  相似文献   

16.
We present new spectroscopy in the optical range and 21-cm H  i data covering the Ruprecht 55 (Ru 55) field in the Puppis window where several authors have proposed the existence of one (or two) clusters.
We have determined new MK spectral types for about 50 stars in the region, finding 43 OB-type stars among them. LS 985 was found to be an O9 V + O9.5 III binary and it is the earliest type of star in our observed sample.
We have identified a stellar OB association (Ru 55), which is most likely related to a depletion detected in our H  i data, as: (i) they are located at the same distance (6 kpc), within observational errors; (ii) both have similar radial velocities (∼67 km s−1); (iii) current OB stars could have provided the energy needed to blow the cavity; (iv) the dynamical time-scale for the hole buildup matches the age estimated for the earliest OB stars; and (v) LS 985 might be responsible for ionizing the H  i cavity inner walls close to it.  相似文献   

17.
K -band spectra of young stellar candidates in four Southern hemisphere clusters have been obtained with the Gemini Near-Infrared Spectrograph in Gemini South. The clusters are associated with IRAS sources that have colours characteristic of ultracompact H  ii regions. Spectral types were obtained by comparison of the observed spectra with those of a near-infrared (NIR) library; the results include the spectral classification of nine massive stars and seven objects confirmed as background late-type stars. Two of the studied sources have K -band spectra compatible with those characteristic of very hot stars, as inferred from the presence of C  iv , N  iii and N  v emission lines at 2.078, 2.116 and 2.100 μm, respectively. One of them, I16177_IRS1, has a K -band spectrum similar to that of Cyg OB2 7, an O3If* supergiant star. The nebular K -band spectrum of the associated Ultra-Compact (UC) H  ii region shows the s-process [Kr  iii ] and [Se  iv ] high excitation emission lines, previously identified only in planetary nebula. One young stellar object was found in each cluster, associated with either the main IRAS source or a nearby resolved Midecourse Space eXperiment ( MSX ) component, confirming the results obtained from previous NIR photometric surveys. The distances to the stars were derived from their spectral types and previously determined JHK magnitudes; they agree well with the values obtained from the kinematic method, except in the case of IRAS  15408−5356  , for which the spectroscopic distance is about a factor of 2 smaller than the kinematic value.  相似文献   

18.
Star clusters are born in a highly compact configuration, typically with radii of less than about 1 pc roughly independently of mass. Since the star formation efficiency is less than 50 per cent by observation and because the residual gas is removed from the embedded cluster, the cluster must expand. In the process of doing so it only retains a fraction f st of its stars. To date there are no observational constraints for f st, although N -body calculations by Kroupa, Aarseth & Hurley suggest it to be about 20–30 per cent for Orion-type clusters. Here we use the data compiled by Testi et al., Testi, Palla & Natta and Testi, Palla & Natta for clusters around young Ae/Be stars and by de Wit et al. and de Wit et al. around young O stars and the study of de Zeeuw et al. of OB associations and combine these measurements with the expected number of stars in clusters with primary Ae/Be and O stars, respectively, using the empirical correlation between maximal stellar mass and star cluster mass of Weidner & Kroupa. We find that   f st < 50  per cent with a decrease to higher cluster masses/more massive primaries. The interpretation would be that cluster formation is very disruptive. It appears that clusters with a birth stellar mass in the range  10–103 M⊙  keep at most 50 per cent of their stars.  相似文献   

19.
Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar‐type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low‐mass stars. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow‐up studies of magnetism among young pre‐main‐sequence stars. We obtained high‐resolution, high signal‐to‐noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb‐Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period Prot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non‐statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current‐driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate‐mass stars could be an alternative to a frozen‐in fossil field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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