Observed Emission Measure Research Articles

The soft X-ray background with Suzaku. I. Milky Way halo

Abstract We present measurements of the soft X-ray background emission for 130 Suzaku observations at 75° < l < 285° and |b| > 15° obtained from 2005 to 2015, covering nearly one solar cycle. In addition to the standard soft X-ray background model consisting of the local hot bubble and the Milky Way Halo (MWH), we include a hot collisional-ionization-equilibrium component with a temperature of ∼0.8 keV to reproduce spectra of a significant fraction of the lines of sight. Then, the scatter in the relation between the emission measure vs. temperature of the MWH component is reduced. Here, we exclude time ranges with high count rates to minimize the effect of the solar wind charge exchange (SWCX). However, the spectra of almost the same lines of sight are inconsistent. The heliospheric SWCX emissions likely contaminate and give a bias in measurements of temperature and the emission measure of the MWH. Excluding the data around the solar maximum and using the data taken before the end of 2009, at |b| > 35° and 105° < l < 255°, the temperature (0.22 keV) and emission measure (2 × 10−3 cm−6 pc) of the MWH are fairly uniform. The increase of the emission measure toward the lower Galactic latitude at |b| < 35° indicates the presence of a disk-like morphology component. A composite model which consists of disk-like and spherical-morphology components also reproduces the observed emission measure distribution of MWH. In this case, the hydrostatic mass at a few tens of kiloparsec from the Galactic center agrees with the gravitational mass of the Milky Way. The plasma with the virial temperature likely fills the Milky Way halo in nearly hydrostatic equilibrium. Assuming a gas metallicity of 0.3 solar, the upper limit of the gas mass of the spherical component out to 250 kpc, or the virial radius, is ∼ a few × 1010 M⊙.

Plasma dynamics in the flaring loop observed by RHESSI

Context. Hard X-rays (HXRs) contain the most direct information about the non-thermal electron population in solar flares. The approximation of the HXR emission mechanism (bremsstrahlung), known as the thick-target model, is well developed. It allows one to diagnose the physical conditions within a flaring structure. The thick-target model predicts that in flare foot points, we should observe lowering of HXR sources’ altitude with increasing energy. Aims. The foot point of HXR sources result from the direct interaction of non-thermal electron beams with plasma in the lower part of the solar atmosphere, where the density increases rapidly. Therefore, we can estimate the plasma density distribution along the non-thermal electron beam directly from the observations of the altitude-energy relation obtained for the HXR foot point sources. However, the relation is not only density-dependent. Its shape is also determined by the power-law distribution of non-thermal electrons. Additionally, during the impulsive phase, the plasma density and a degree of ionisation within foot points may change dramatically due to heating and chromospheric evaporation. For this reason, the interpretation of observed HXR foot point sources’ altitudes is not straightforward and needs detailed numerical modelling of the electron precipitation process. Methods. We present the results of numerical modelling of one well-observed solar flare. We used HXR observations obtained by RHESSI. The numerical model was calculated using the hydrodynamic 1D model with an application of the Fokker-Planck formalism for non-thermal beam precipitation. Results. HXR data were used to trace chromospheric density changes during a non-thermal emission burst, in detail. We have found that the amount of mass that evaporated from the chromosphere is in the range of 2.7 × 1013 − 4.0 × 1014 g. This is in good agreement with the ranges obtained from hydrodynamical modelling of a flaring loop (2.3 × 1013 − 3.3 × 1013 g), and from an analysis of observed emission measure in the loop top (3.9 × 1013 − 5.3 × 1013 g). Additionally, we used specific scaling laws which gave another estimation of the evaporated mass around 2 × 1014 g. Conclusions. Consistency between the obtained values shows that HXR images may provide an important constraint for models – a mass of plasma that evaporated due to a non-thermal electron beam depositing energy in the chromosphere. High-energy, non-thermal sources’ (above 20 keV in this case) positions fit the column density changes obtained from the hydrodynamical model perfectly. Density changes seem to be less affected by the electrons’ spectral index. The obtained results significantly improve our understanding of non-thermal electron beam precipitation and allow us to refine the energy balance in solar flare foot points during the impulsive phase.

Understanding Heating in Active Region Cores through Machine Learning. II. Classifying Observations

Abstract Constraining the frequency of energy deposition in magnetically closed active region cores requires sophisticated hydrodynamic simulations of the coronal plasma and detailed forward modeling of the optically thin line-of-sight integrated emission. However, understanding which set of model inputs best matches a set of observations is complicated by the need for any proposed heating model to simultaneously satisfy multiple observable constraints. In this paper, we train a random forest classification model on a set of forward-modeled observable quantities, namely the emission measure slope, the peak temperature of the emission measure distribution, and the time lag and maximum cross-correlation between multiple pairs of AIA channels. We then use our trained model to classify the heating frequency in every pixel of active region NOAA 1158 using the observed emission measure slopes, peak temperatures, time lags, and maximum cross-correlations, and are able to map the heating frequency across the entire active region. We find that high-frequency heating dominates in the inner core of the active region while intermediate-frequency dominates closer to the periphery of the active region. Additionally, we assess the importance of each observed quantity in our trained classification model and find that the emission measure slope is the dominant feature in deciding with which heating frequency a given pixel is most consistent. The technique presented here offers a very promising and widely applicable method for assessing observations in terms of detailed forward models given an arbitrary number of observable constraints.

The Distribution of Time Delays Between Nanoflares in Magnetohydrodynamic Simulations

Nanoflares are thought to be an important energy source for heating the solar corona. The exact amount of heating contributed by nanoflares depends, in part, on the frequency of the heating and the time interval between nanoflares. Several numerical models have attempted to constrain the frequency of the heating by fitting to observed emission measures. To date, however, no physically motivated value for the time interval between nanoflares has been obtained. In this paper, we calculate a physically motivated distribution of time intervals between successive “nanoflare” reconnection events in driven magnetohydrodynamic simulations. We show that this distribution follows a power law with a slope near −1, much shallower than previously inferred from observations and determined from loop model comparisons with observations. We show that the energy flux injected into the corona in our model is of order $10^{7}~\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$, and that the heating rate due to these reconnection events reaches values of order $1\mbox{--}10~\text{erg}\,\text{cm}^{-3}\,\text{s}^{-1}$. Additionally, we show that this power law slope is dependent only weakly on the amount of magnetic helicity injected into the solar magnetic field by the photospheric motions, but the rate of reconnection is about $45\%$ higher if no magnetic helicity is injected.

The Diffuse Ionized Gas Halo of the Small Magellanic Cloud

Abstract Observations with the Wisconsin Mapper reveal a large, diffuse ionized halo that surrounds the Small Magellanic Cloud (SMC). We present the first kinematic survey of an extended region around the galaxy, from to and covering . The ionized gas emission extends far beyond the central stellar component of the galaxy, reaching similar distances to that of the diffuse neutral halo traced by 21 cm observations. emission extends several degrees beyond the sensitivity of current H i surveys toward smaller galactic longitudes and more negative galactic latitudes. The velocity field of the ionized gas near the SMC appears similar to the neutral halo of the galaxy. Using the observed emission measure as a guide, we estimate the mass of this newly revealed ionized component to be roughly , which is comparable to the total neutral mass in the same region of . We find ratios of the total ionized gas mass divided by the total neutral plus ionized gas mass in three distinct subregions to be: (1) 46%–54% throughout the SMC and its extended halo, (2) 12%–32% in the SMC Tail that extends toward the Magellanic Bridge, and (3) 65%–79% in a filament that extends away from the SMC toward the Magellanic Stream. This newly discovered, coherent filament does not appear to have a well-structured neutral component and is also not coincident with two previously identified filaments traced by 21 cm emission within the Stream.

A new method to probe the thermal electron content of the Galaxy through spectral analysis of background sources

We present a new method for probing the thermal electron content of the Galaxy by spectral analysis of background point sources in the absorption-only limit to the radiative transfer equation. In this limit, calculating the spectral index, $\alpha$, of these sources using a natural logarithm results in an additive factor, which we denote $\alpha_\mathrm{EM}$, resulting from the absorption of radiation due to the Galactic thermal electron population. We find that this effect is important at very low frequencies ($\nu\lesssim200$ MHz), and that the frequency spacing is critical. We model this effect by calculating the emission measure across the sky. A (smooth) thermal electron model for the Galaxy does not fit the observed emission measure distribution, but a simple, cloud-based model to represent the clumpy nature of the warm interstellar medium does. This model statistically reproduces the Galactic emission measure distribution as obtained independently from $H_\alpha$ data well. We find that at the lowest frequencies ($\sim10-50$ MHz), the observed spectral index for a large segment of the Galaxy below Galactic latitudes of $\lesssim15^\circ$ could be changed significantly (i.e., $\alpha_\mathrm{EM}\gtrsim0.1$). This method therefore provides a correction to low-frequency spectral index measurements of extragalactic sources, and provides a sensitive probe of the thermal electron distribution of the Galaxy using current and next-generation low-frequency radio telescopes. We show that this effect should be robustly detectable individually in the strongest sources, and statistically in source samples at a level of $\alpha_\mathrm{EM}\gtrsim0.18,0.06$, and 0.02 for source densities of 10, 100 and 1,000 sources per square degree.

ANXMM-NEWTONSURVEY OF THE SOFT X-RAY BACKGROUND. III. THE GALACTIC HALO X-RAY EMISSION

We present measurements of the Galactic halo's X-ray emission for 110 XMM-Newton sight lines, selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on ~4/5 of our sight lines. The temperature is fairly uniform (median = 2.22e6 K, interquartile range = 0.63e6 K), while the emission measure and intrinsic 0.5--2.0 keV surface brightness vary by over an order of magnitude (~(0.4-7)e-3 cm^-6 pc and ~(0.5-7)e-12 erg cm^-2 s^-1 deg^-2, respectively, with median detections of 1.9e-3 cm^-6 pc and 1.5e-12 erg cm^-2 s^-1 deg^-2, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halo morphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper.

BRIGHT POINTS: MULTITHERMAL ANALYSIS AS A TEST OF STEADY HEATING MODELS

X-ray bright points are small, million-degree features in the solar atmosphere composed of short coronal loops. They are magnetically driven structures associated with photospheric magnetic bipoles. Their relatively small size and simple structure suggest they are ideal candidates for comparisons with coronal heating models. In this paper, we present the analysis of 12 bright points using data from the EUV Imaging Spectrometer on Hinode and the Michelson Doppler Imager on Solar and Heliospheric Observatory. Using the spectroscopy data, we construct differential emission measure (DEM) curves, calculate the electron density, and find DEM-weighted temperatures. In addition, we determine the most likely ionization balance. Using the magnetic field observations, we complete potential field extrapolations of the magnetograms and estimate the loop lengths. Using this information, we construct models assuming the bright points are formed of hundreds of strands, each heated steadily and uniformly. We formulate the models so that the observed emission measure distribution is matched within a few percent. We then compare the densities determined from the models, (1.4-5.0) × 109, to those calculated from spectral data, (0.6-2.0) × 109. We find the majority of bright points do not agree with steady uniform heating models; instead they are underdense relative to their expected density by a factor of 0.16-0.82.

A MODEL FOR THE ORIGIN OF HIGH DENSITY IN LOOPTOP X-RAY SOURCES

Super-hot looptop sources, detected in some large solar flares, are compact sources of HXR emission with spectra matching thermal electron populations exceeding 30 megakelvins. High observed emission measure, as well as inference of electron thermalization within the small source region, both provide evidence of high densities at the looptop; typically more than an order of magnitude above ambient. Where some investigators have suggested such density enhancement results from a rapid enhancement in the magnetic field strength, we propose an alternative model, based on Petschek reconnection, whereby looptop plasma is heated and compressed by slow magnetosonic shocks generated self-consistently through flux retraction following reconnection. Under steady conditions such shocks can enhance density by no more than a factor of four. These steady shock relations (Rankine-Hugoniot relations) turn out to be inapplicable to Petschek's model owing to transient effects of thermal conduction. The actual density enhancement can in fact exceed a factor of ten over the entire reconnection outflow. An ensemble of flux tubes retracting following reconnection at an ensemble of distinct sites will have a collective emission measure proportional to the rate of flux tube production. This rate, distinct from the local reconnection rate within a single tube, can be measured separately through flare ribbon motion. Typical flux transfer rates and loop parameters yield emission measures comparable to those observed in super-hot sources.

OBSERVING THE ROOTS OF SOLAR CORONAL HEATING—IN THE CHROMOSPHERE

The Sun's corona is millions of degrees hotter than its 5,000 K photosphere. This heating enigma is typically addressed by invoking the deposition at coronal heights of non-thermal energy generated by the interplay between convection and magnetic field near the photosphere. However, it remains unclear how and where coronal heating occurs and how the corona is filled with hot plasma. We show that energy deposition at coronal heights cannot be the only source of coronal heating, by revealing a significant coronal mass supply mechanism that is driven from below, in the chromosphere. We quantify the asymmetry of spectral lines observed with Hinode and SOHO and identify faint but ubiquitous upflows with velocities that are similar (50-100 km/s) across a wide range of magnetic field configurations and for temperatures from 100,000 to several million degrees. These upflows are spatio-temporally correlated with and have similar upward velocities as recently discovered, cool (10,000 K) chromospheric jets or (type II) spicules. We find these upflows to be pervasive and universal. Order of magnitude estimates constrained by conservation of mass and observed emission measures indicate that the mass supplied by these spicules can play a significant role in supplying the corona with hot plasma. The properties of these events are incompatible with coronal loop models that only include nanoflares at coronal heights. Our results suggest that a significant part of the heating and energizing of the corona occurs at chromospheric heights, in association with chromospheric jets.

The Effects of Clumps in Explaining X‐Ray Emission Lines from Hot Stars

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.

RHESSI Microflares: II. Implications for Loop Structure and Evolution

We present simple analytic models which predict the peak X-ray emission measure and temperature attained in flares in which the chromospheric evaporation process takes place either in a single ‘monolithic’ loop or in a loop consisting of several filaments that are created successively as the energy release process proceeds in time. As possible mechanisms driving chromospheric evaporation we consider both classical heat conduction from the loop top and non-thermal electron beams. The model predictions are tested for a set of 18 well studied RHESSI microflares. The results suggest beam driven evaporation in filamented loops as being capable of accounting for the observed emission measures and temperatures though there are issues with the very high beam densities needed. On the other hand, estimates of the emission measures achieved by conductive evaporation which are derived by using the Rosner – Tucker – Vaiana (RTV) scaling law are much larger than the observed ones. Possible reasons for this discrepancy are discussed.

Prominence modelling: from observed emission measures to temperature profiles

Aims. We outline the construction of prominence - corona transition region models based upon the observations of one particular prominence. Methods. The differential emission measure curves from observations were approximated by simple analytical functions. On this basis we constructed the temperature curve and calculated the radiative losses, the gains by thermal conduction, and some estimates for the wave heating. Results. The temperature curve was calculated in the range between 23 000 K and 450 000 K. The resulting transition region can be divided into an inner region where the temperature is low and the radiative losses are very large, a part with a very steep temperature rise from 40 000 K to around 250 000 K over a width of only 500 km, and an extended high temperature region. Both the conductive heating and our estimates for a possible wave cooling/heating peak very sharply in the region with the very large temperature gradient. The consequences for the energy balance are discussed.

A Transient Heating Model for the Structure and Dynamics of the Solar Transition Region

Understanding the structure and dynamics of the Sun's transition region has been a major challenge to scientists since the Skylab era. In particular, the characteristic shape of the emission measure distribution and the Doppler shifts observed in EUV emission lines have thus far resisted all theoretical and modeling efforts to explain their origin. Recent observational advances have revealed a wealth of dynamic fine-scale structure at transition-region temperatures, validating earlier theories about the existence of such cool structure and explaining in part why static models focusing solely on hot, large-scale loops could not match observed conditions. In response to this newly confirmed picture, we have investigated numerically the hydrodynamic behavior of small, cool magnetic loops undergoing transient heating spatially localized near the chromospheric footpoints. For the first time we have successfully reproduced both the observed emission measure distribution over the entire range log T = 4.7-6.1 and the observed temperature dependence of the persistent redshifts. The closest agreement between simulations and observations is obtained with heating timescales of the order of 20 s every 100 s, a length scale of the order of 1 Mm, and energy deposition within the typical range of nanoflares. We conclude that small, cool structures can indeed produce most of the quiet solar EUV output at temperatures below 1 MK.

Spatially resolved X-ray spectroscopy of cooling clusters of galaxies

We present spatially resolved X-ray spectra taken with the EPIC cameras of XMM-Newton of a sample of 17 cooling clusters and three non-cooling clusters for comparison. The deprojected spectra are analyzed with a multi-temperature model, independent of any a priori assumptions about the physics behind the cooling and heating mechanisms. All cooling clusters show a central decrement of the average temperature, most of them of a factor of . Three clusters (Sérsic 159-3, MKW 3s and Hydra A) only show a weak temperature decrement, while two others (A 399 and A 2052) have a very strong temperature decrement. All cooling clusters show a weak pressure gradient in the core. More important, at each radius within the cooling region the gas is not isothermal. The differential emission measure distribution shows a strong peak near the maximum (ambient) temperature, with a steep decline towards lower temperatures, approximately proportional to T3, or alternatively a cut-off at about a quarter to half of the maximum temperature. In general, we find a poor correlation between radio flux of the central galaxy and the temperature decrement of the cooling flow. This is interpreted as evidence that except for a few cases (like the Hydra A cluster) heating by a central AGN is not the most common cause of weak cooling flows. We investigate the role of heat conduction by electrons and find that the theoretically predicted conductivity rates are not high enough to balance radiation losses. The differential emission measure distribution has remarkable similarities with the predictions from coronal magnetic loop models. Also the physical processes involved (radiative cooling, thermal conduction along the loops, gravity) are similar for clusters loops and coronal loops. If coronal loop models apply to clusters, we find that a few hundred loops per scale height should be present. The typical loop sizes deduced from the observed emission measure distribution are consistent with the characteristic magnetic field sizes deduced from Faraday rotation measurements.

On the Nature of the “Moss” Observed byTRACE

"Moss" is the name given to low-lying (~3000 km), hot (~1 MK) solar coronal plasma that has been observed recently by the Transition Region and Coronal Explorer (TRACE). This paper investigates two hypotheses regarding the nature of the moss: (1) emission from small, million degree loops; (2) emission from the legs of 3-10 million degree loops. We update the coronal radiative loss curve, using the most recent results for coronal abundances, and use an analytical loop model to find that the first hypothesis requires a filling factor close to unity to reproduce the observed emission measure, while the second hypothesis results in a filling factor of about 0.1, in agreement with other independent multiwavelength analyses of moss. We find that the vertical extent and the height of the moss layer above the limb are also very well reproduced with the second hypothesis. We further show that the observed brightness of the moss scales linearly with the loop pressure and filling factor, independent of the loop length, and we derive a general expression for the conversion factor.

The Escape of Ionizing Photons from the Galaxy

The Magellanic Stream and several high velocity clouds have now been detected in optical line emission. The observed emission measures and kinematics are most plausibly explained by photoionization due to hot, young stars in the Galactic disk. The highly favorable orientation of the Stream allows an unambiguous determination of the fraction of ionizing photons, F_esc, which escape the disk. We have modelled the production and transport of ionizing photons through an opaque interstellar medium. Normalization to the Stream detections requires F_esc = 6%, in reasonable agreement with the flux required to ionize the Reynolds layer. Neither shock heating nor emission within a hot Galactic corona can be important in producing the observed H-alpha emission. If such a large escape fraction is typical of L_* galaxies, star-forming systems dominate the extragalactic ionizing background. Within the context of this model, both the three-dimensional orientation of the Stream and the distances to high-velocity clouds can be determined by sensitive H-alpha observations.

Ionized Gas in Sagittarius B2 Main on Scales of 0.065 Arcsecond (600 AU)

The ionized gas in the Sagittarius B2 Main star-forming region has been imaged with ~65 mas (600 AU) resolution using the Very Large Array (VLA) at 7 mm in the A configuration. These observations have ~4-5 times the resolution of the previous highest resolution images of this source. These VLA observations show that the central four ultracompact H II (UC H II) regions in Sgr B2 Main (F1, F2, F3, and F4) break up into ~20 separate UC H II regions. The individual sources have small resolved sizes (~10-3 pc), high derived emission measures (EM ~ 109 pc cm-6), and high rms electron densities (ne ~ 106 cm-3). We compare the observed radius and derived emission measure of each of these regions with those expected for a UC H II region in pressure equilibrium with its environment, using previously published temperatures and densities for the Sgr B2 molecular core (nH2=2×107 cm-3). We find that the observed emission measures are in fact higher than those predicted in a simple pressure-confinement model, suggesting that the molecular densities on scales that confine the UC H II regions may be higher than 2 × 107 cm-3.

The Galactic Halo Ionising Field

AbstractThere has been much debate in recent decades as to what fraction of ionising photons from star-forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4 m and the AAT 3·9 m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. We present a simple model to demonstrate that, while the distance to the Magellanic Stream is uncertain, the observed emission measures (εm ≈ 0·5 – 1 cm−6 pc) are most plausibly explained by photoionisation due to hot, young stars. This model requires that the mean Lyman-limit opacity perpendicular to the disk is τLL ≈ 3, and the covering fraction of the resolved clouds is close to unity. Alternative sources (e.g. shock, halo, LMC or metagalactic radiation) contribute negligible ionising flux.

X-ray emission from near-main-sequence B stars

view Abstract Citations (133) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS X-Ray Emission from Near--Main-Sequence B Stars Cassinelli, J. P. ; Cohen, D. H. ; Macfarlane, J. J. ; Sanders, W. T. ; Welsh, B. Y. Abstract Results of a Roentgen Satellite (ROSAT) X-ray survey of 12 nearby near-main-sequence B stars are presented. Objects with very low interstellar hydrogen column density were chosen to study the soft X-ray emission properties of the stars. All of the stars were detected at the 3 sigma level. Spectral fits to seven of the stars are presented, and temperatures and source emission measures are derived. The spectra and characterized by emission from gas at a temperature of about 2 x 106 K, which is lower than that typically observed for O stars. The ratio Lx/LBol decreases sharply with spectral types later than B1, reaching values of about 10-9 at B3 V; much less than the value of 10-7 that holds for O stars and early-B stars. The survey includes four beta Cephei stars and two Be stars. Our observations are consistent with the finding that beta Cephei variables have softer spectra than other stars of the same spectral type. The X-ray emission measures for the stars are compared with upper and lower limits derived using mass loss rates and terminal velocities from line-driven wind theory. It is concluded that if shocks embedded in the winds are the source of the X-ray emission then a significant fraction of the winds in B stars must be hot. One to a few shocks can account for the observed emission measures. As a function of spectral type, there appears to be a transition in X-ray emission properties at about B1 to B1.5, with later type stars having a smaller Lx/LBol ratio. Coronal and shock models are discussed in regards to the BO V star tau Sco which has an anomalously high source temperature. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173683 Bibcode: 1994ApJ...421..705C Keywords: B Stars; Cepheid Variables; Emission Spectra; Main Sequence Stars; Stellar Mass; X Ray Astronomy; X Ray Spectra; Astronomical Spectroscopy; Hydrogen; Interstellar Gas; O Stars; Rosat Mission; Shock Waves; Spaceborne Astronomy; Astronomy; SHOCK WAVES; STARS: EARLY-TYPE; STARS: MASS LOSS; X-RAYS: STARS full text sources ADS | data products SIMBAD (12)