The Correlation Between X-Ray Line Ionization and Optical Spectral Types of the Ob Stars

Walborn, N. R.; Nichols, Joy S.; Waldron, W. L.

doi:10.1088/0004-637x/703/1/633

Cited by 29 publications

(44 citation statements)

References 61 publications

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“…These profiles exhibit Discrete Absorption features at 1000-1100 km s −1 and edge velocites of -1800 km s −1 (Doazan 1982;Kaper et al 1996;Smith et al 1998a,b). Our analysis shows the presence of soft X-ray components whose temperatures, fluxes, and emission measures (Table 1) are consistent with those of normal massive stars reported by Walborn et al (2009), for example. It is thus expected that at least part of such emission is emanating from the radiatively driven wind of γ Cas.…”

Section: Velocity Broadening and Shifts In Warm Plasma Linessupporting

confidence: 90%

γCassiopeiae: an X-ray Be star with personality

Oliveira

Smith

Motch

2010

A&A

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An exciting unsolved problem in the study of high energy processes of early type stars concerns the physical mechanism for producing X-rays near the Be star γ Cassiopeiae. By now we know that this source and several "γ Cas analogs" exhibit an unusual hard thermal Xray spectrum, compared both to normal massive stars and the non-thermal emission of known Be/X-ray binaries. Also, its light curve is variable on almost all conceivable timescales. In this study we reanalyze a high dispersion spectrum obtained by Chandra in 2001 and combine it with the analysis of a new (2004) spectrum and light curve obtained by XMM-Newton. We find that both spectra can be fit well with 3-4 optically thin, thermal components consisting of a hot component having a temperature kT Q ∼ 12-14 keV, perhaps one with a value of ∼2.4 keV, and two with well defined values near 0.6 keV and 0.11 keV. We argue that these components arise in discrete (almost monothermal) plasmas. Moreover, they cannot be produced within an integral gas structure or by the cooling of a dominant hot process. Consistent with earlier findings, we also find that the Fe abundance arising from K-shell ions is significantly subsolar and less than the Fe abundance from L-shell ions. We also find novel properties not present in the earlier Chandra spectrum, including a dramatic decrease in the local photoelectric absorption of soft X-rays, a decrease in the strength of the Fe and possibly of the Si K fluorescence features, underpredicted lines in two ions each of Ne and N (suggesting abundances that are ∼1.5-3× and ∼4× solar, respectively), and broadening of the strong Ne X Lyα and O VIII Lyα lines. In addition, we note certain traits in the γ Cas spectrum that are different from those of the fairly well studied analog HD 110432 -in this sense the stars have different "personalities." In particular, for γ Cas the hot X-ray component remains nearly constant in temperature, and the photoelectric absorption of the X-ray plasmas can change dramatically. As found by previous investigators of γ Cas, changes in flux, whether occurring slowly or in rapidly evolving flares, are only seldomly accompanied by variations in hardness. Moreover, the light curve can show a "periodicity" that is due to the presence of flux minima that recur semiregularly over a few hours, and which can appear again at different epochs.

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Section: Velocity Broadening and Shifts In Warm Plasma Linessupporting

confidence: 90%

γCassiopeiae: an X-ray Be star with personality

Oliveira

Smith

Motch

2010

A&A

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“…The rotation period of this star is still uncertain: Oskinova et al (2001) studied ASCA observations that covered just the expected rotation period of the star of about 0.8-1.0 d. A X-ray flux variability with ∼20% amplitude in the ASCA passband (0.5-10 keV) with a period of 0.77 d was reported. The X-ray spectra of ζ Oph are not especially hard as found from Chandra HETGS and Suzaku data (Waldron & Cassinelli 2007;Walborn et al 2009). The variation period is also not well-defined in our observations.…”

Section: ζ Ophmentioning

confidence: 60%

“…The X-ray luminosity based on Chandra observations is log (L X ) = 31.82 and the ratio of X-ray to bolometric luminosity log (L X /L bol ) = −6.63 is slightly above the average (Ramirez et al 2004;Nazé 2009), as may be expected from a magnetic star. The high-resolution X-ray spectrum of 15 Mon is typical for a star with its spectral type and, apparently, not especially hard (Walborn et al 2009). The previously measured magnetic field using the full FORS 1 spectrum coverage was of positive polarity at a 2.6σ significance level (Hubrig et al 2009a).…”

Section: Hd 36879mentioning

confidence: 98%

Exploring the origin of magnetic fields in massive stars

Hubrig

Schöller

Ilyin

et al. 2013

A&A

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Context. Theories on the origin of magnetic fields in massive stars remain poorly developed, because the properties of their magnetic field as function of stellar parameters could not yet be investigated. Additional observations are of utmost importance to constrain the conditions that are conducive to magnetic fields and to determine first trends about their occurrence rate and field strength distribution. Aims. To investigate whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 67 new spectropolarimetric observations for 30 massive stars. Among the observed sample, roughly one third of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Methods. Spectropolarimetric observations were obtained during four different nights using the low-resolution spectropolarimetric mode of FOcal Reducer low dispersion Spectrograph (FORS 2) mounted on the 8-m Antu telescope of the VLT. Furthermore, we present a number of follow-up observations carried out with the high-resolution spectropolarimeters SOFIN mounted at the Nordic Optical Telescope (NOT) and HARPS mounted at the ESO 3.6 m between 2008 and 2011. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. Results. The presence of a magnetic field is confirmed in nine stars previously observed with FORS

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“…Our fits provide estimates of the circumstellar absorption, but its correlation with hardness ratio again seems quite shallow and scattered, thus not formally significant (Figure 6), and as already mentioned in Section 2.1, there is no trend whatsoever between this additional column and the bolometric luminosity. Second, Walborn et al (2009) detected (1) a larger ionization of the plasma in early-type stars (implying a hotter plasma) and (2) a shift of the overall spectral distribution toward lower energies for late-type stars. These trends, if confirmed, would also be compatible with a higher hardness ratio toward larger bolometric luminosities.…”

Section: Hardness Ratiosmentioning

confidence: 98%

Global X-Ray Properties of the O and B Stars In Carina

Nazé¹,

Broos²,

Oskinova³

et al. 2011

ApJS

124

160

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The key empirical property of the X-ray emission from O stars is a strong correlation between the bolometric and X-ray luminosities. In the framework of the Chandra Carina Complex Project, 129 O and B stars have been detected as X-ray sources; 78 of those, all with spectral type earlier than B3, have enough counts for at least a rough X-ray spectral characterization. This leads to an estimate of the L X -L BOL ratio for an exceptional number of 60 O stars belonging to the same region and triples the number of Carina massive stars studied spectroscopically in X-rays. The derived log(L X /L BOL ) is −7.26 for single objects, with a dispersion of only 0.21 dex. Using the properties of hot massive stars listed in the literature, we compare the X-ray luminosities of different types of objects. In the case of O stars, the L X -L BOL ratios are similar for bright and faint objects, as well as for stars of different luminosity classes or spectral types. Binaries appear only slightly harder and slightly more luminous in X-rays than single objects; the differences are not formally significant (at the 1% level), except for the L X -L BOL ratio in the medium (1.0-2.5 keV) energy band. Weak-wind objects have similar X-ray luminosities but they display slightly softer spectra compared with "normal" O stars with the same bolometric luminosity. Discarding three overluminous objects, we find a very shallow trend of harder emission in brighter objects. The properties of the few B stars bright enough to yield some spectral information appear to be different overall (constant X-ray luminosities, harder spectra), hinting that another mechanism for producing X-rays, besides wind shocks, might be at work. However, it must be stressed that the earliest and X-ray brightest among these few detected objects are similar to the latest O stars, suggesting a possibly smooth transition between the two processes.

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The Correlation Between X-Ray Line Ionization and Optical Spectral Types of the Ob Stars

Cited by 29 publications

References 61 publications

γCassiopeiae: an X-ray Be star with personality

γCassiopeiae: an X-ray Be star with personality

Exploring the origin of magnetic fields in massive stars

Global X-Ray Properties of the O and B Stars In Carina

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