arXiv subjects

E. D. Feigelson

Publications and source records attributed to E. D. Feigelson.

At least 37 records · Page 2Linked to original sources

arXiv2005

Parsec-scale X-ray Flows in High-mass Star-forming Regions

The Chandra X-ray Observatory is providing remarkable new views of massive star-forming regions, revealing all stages in the life cycle of high-mass stars and their effects on their surroundings. We present a Chandra tour of several high-mass star-forming regions, highlighting physical processes that characterize the life of a cluster of high-mass stars, from deeply-embedded cores too young to have established an HII region to superbubbles so large that they shape our views of galaxies. Along the way we see that X-ray observations reveal hundreds of stellar sources powering great HII region complexes, suffused by both hard and soft diffuse X-ray structures caused by fast O-star winds thermalized in wind-wind collisions or by termination shocks against the surrounding media. Finally, we examine the effects of the deaths of high-mass stars that remained close to their birthplaces, exploding as supernovae within the superbubbles that these clusters created. We present new X-ray results on W51 IRS2E and 30 Doradus and we introduce new data on Trumpler 14 in Carina and the W3 HII region complexes W3 Main and W3(OH).

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arXiv2005

X-Ray Flares of Sun-Like Young Stellar Objects and Their Effects on Protoplanetary Disks

Astronomical observations of flares from analogs of the early Sun have the potential to give critical insights into the high energy irradiation environment of protoplanetary disks. Solar-mass young stellar objects are significantly more X-ray luminous than the typical low-mass T Tauri star. They undergo frequent strong flaring on a several day time scale. Very powerful flares also occur, but on a longer time frame. The hard X-ray spectrum of these stars become even harder during flaring. The X-rays from these sun-like young stellar objects have the potential to ionize circumstellar material out to large distances. Three specific illustrations are given of the effects of the X-rays: The physics and chemistry of the atmospheres of the inner accretion disks; the ionization level at the disk midplane, important for the viability of the magnetorotational instability; and the nuclear fluence in the irradiation zone just interior to the inner edge of the disk, important in local irradiation scenarios for producing the short-lived radionuclides found in meteorites.

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arXiv2005

Rotational modulation of X-ray emission in Orion Nebula young stars

We investigate the spatial distribution of X-ray emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light-curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17x17 arcmin field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period while in seven cases it is about half that value, possibly due to the presence of X-ray emitting structures at opposite stellar longitudes. These results constitute the largest sample of low mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (L_X/L_bol ~ 10^(-3)) showing that saturation is not due to the filling of the stellar surface with X-ray emitting regions.

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arXiv2005

Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic fields?

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 days observation of the Orion Nebula Cluster (ONC). Analysis of the flare decay allows to determine the size, peak density and magnetic field of the flaring structure. A total of 32 events (the most powerful 1% of COUP flares), have sufficient statistics for the analysis. A broad range of decay times (from 10 to 400 ks) are present in the sample. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures which are found, are in a number of cases very long, with semi-lengths up to 10^12 cm, implying the presence of magnetic fields of hundreds of G extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures ($ >> R_*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L <= R_*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures which confine the flaring plasma are actually the same type of structures which channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

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arXiv2005

X-ray emission from early-type stars in the Orion Nebula Cluster

The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Only two of the massive stars show exclusively the constant soft-spectrum emission expected from the standard model for X-ray emission from hot stars involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity, suggesting magnetic confinement of winds with large-scale shocks. Most of the stars in the weak-wind class exhibit X-ray flares and have luminosities that are consistent with magnetic activity from known or unseen low-mass companions. All non-detections belong to the weak-wind class.

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arXiv2005

Membership of the Orion Nebula Population from the Chandra Orion Ultradeep Project

The Chandra Orion Ultradeep project (COUP) observation described in a companion paper by Getman et al. provides an exceptionally deep X-ray survey of the Orion Nebula Cluster and associated embedded young stellar objects. Membership of the region is important for studies of the stellar IMF, cluster dynamics, and star formation. The COUP study detected 1616 X-ray sources. In this study we confirm cloud membership for 1315 stars, identify 16 probable foreground field stars having optical counterparts with discrepant proper motions, and classify the remaining 285 X-ray sources, of which 51 are lightly and 234 heavily obscured. The 51 lightly obscured sources without known counterparts fall into three groups. (i) Sixteen are likely new members of the Orion Nebula Cluster. (ii) Two with unusually soft and non-flaring X-ray emission appear to be associated with nebular shocks, and may be new examples of X-rays produced at the bow shocks of Herbig-Haro outflows. (iii) The remaining thirty three are very weak uncertain sources, possibly spurious. Out of 234 heavily absorbed sources without optical or near-infrared counterparts 75 COUP sources are likely new embedded cloud members (with membership for 42 confirmed by powerful X-ray flares), and the remaining 159 are likely extragalactic AGN seen through the molecular cloud, as argued by a careful simulation of the extragalactic background population. Finally, a few new binary companions to Orion stars may have been found, but most cases of proximate COUP sources can be attributed to chance superpositions in this crowded field.

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arXiv2005

COUP census of X-ray stars in BN-KL and OMC-1S

We present a study of the X-ray sources detected in the vicinity of two density peaks in the Orion Molecular Cloud 1 (OMC-1) behind the Orion Nebula Cluster (ONC), as seen in the exceptionally deep (~10 days) exposure of the Chandra Orion Ultradeep Project (COUP). We focus on a 40"*50" region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL) and a 60"*75" region around OMC-1S, a secondary star-forming peak some 90" south of BN-KL. Forty-three and sixty X-ray sources were detected in BN-KL and OMC-1S, respectively, of which half and one-third, respectively, were found to be foreground members of the ONC, while the remaining sources are identified as obscured X-ray sources with column densities 22<log(NH)/cm^{-2})<24. All but 5 and 18 of these sources have near-infrared stellar counterparts, and 22 of these appear to be young stellar objects (YSOs). X-ray sources are found close to four luminous mid-IR sources namely BN, IRc3-i2, IRc2-C, and Source n; their X-ray variability and spectral properties are typical of coronal activity in low-mass stars rather than wind emission from massive stars, suggesting that the X-ray emission may be arising from companions. The X-ray light curve of the X-ray source close to BN shows a periodicity of ~8.3 days and from an X-ray image deconvolution of the region around BN, we conclude that either BN itself or a low mass companion with a projected separation of ~200 AU was detected. On the other hand, no emission is seen from the bright radio Source I, held by some to be the main source of luminosity in BN-KL. In OMC-1S, Chandra unveils a new subcluster of seven YSOs without infrared counterparts (abridged).

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arXiv2005

X-rays, pre-main sequence stars and planet formation

The study of magnetic activity of pre-main sequence (PMS) stars through their X-ray emission is entering a mature phase. We review here two recent developments. First, we present some early findings from the Chandra Orion Ultradeep Project (COUP) relating to the age-activity-rotation relations of PMS stars. COUP is a nearly continuous exposure of the Orion Nebula Cluster over 13 days and gives the most comprehensive view to date of X-ray emission from the PMS population. We find that the activity-rotation connection that dominates main sequence activity behavior is absent in PMS stars, although the activity-age relation is present as well as links to stellar mass and volume. Second, we discuss investigations of effects of PMS X-rays on their cold protoplanetary disks. It is possible that the the ionization, heating and spallogenic effects of high energy photons and particles produced in PMS stellar flares have significant effects on disk dynamics, heating and chemistry. X-ray ionization may induce MHD turbulence in the disk which, in turn, may regulate disk evolution and planet formation processes such as Jovian planet migration.

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arXiv2004

Iron Fluorescent Line Emission from Young Stellar Objects in the Orion Nebula

We present the result of a systematic search for the iron Kalpha fluorescent line at 6.4 keV among 1616 X-ray sources detected by ultra-deep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultra-deep Project (COUP). Seven sources are identified to have an excess emission at 6.4 keV among 127 control sample sources with significant counts in the 6.0-9.0 keV band. These seven sources are young stellar objects (YSOs) characterized by intense flare-like flux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the fluorescence by interstellar or circumstellar matter along the line of sight. The X-ray spectral fits and NIR colors of the 6.4 keV sources show that these sources have X-ray absorption of > 1x10^22 cm^(-2) and NIR excess emission, which is not expected when the fluorescence occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises from reflection off of circumstellar disks, which are irradiated by the hard X-ray continuum emission of magnetic reconnection flares.

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arXiv2004

Chandra Orion Ultradeep Project: Observations and Source Lists

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.

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arXiv2004

The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center

We examine the X-ray spectra and variability of the sample of X-ray sources with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2--8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma=0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT>8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.

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arXiv2004

Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center

We examine the spectrum of diffuse emission detected in the 17' by 17' field around Sgr A* during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with originating in a two-temperature plasma, as well as a prominent low-ionization Fe line. The cooler, kT=0.8 keV plasma differs in surface brightness across the image by a factor of 9. This soft plasma is probably heated by supernovae. The radiative cooling rate of the plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 300,000 y. The hotter, kT=8 keV component is more spatially uniform, ranging over a factor of 2 in surface brightness. The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because supernova remnants are not observed to produce thermal plasma hotter than kT=3 keV. Moreover, a kT=8 keV plasma is too hot to be bound to the Galactic center, and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy is ~10^40 erg/s, which corresponds to the entire kinetic energy of one supernova every 3000 y. This rate is unreasonably high. However, alternative explanations for the kT=8 keV diffuse emission are equally unsatisfying. We are left to conclude that either the diffuse emission is heated by an unanticipated source of energy, or that a population of faint (< 10^31 erg/s), hard X-ray sources that are a factor of 10 more numerous than CVs remains to be discovered. (Abridged)

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arXiv2004

The Chandra Deep Field-North Survey: XVII. Evolution of magnetic activity in old late-type stars

The extremely sensitive Chandra Deep Field-North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spindown of stellar rotation from a magnetized stellar wind. Eleven X-ray sources constitute a well-defined sample of 2 G, 2 K0-K4, and 7 M2-M5 stars with median distance around 300 pc. X-ray luminosities are typically log Lx ~ 27 erg/s and is dominated by flares rather than quiescent coronal emission. Models of the population indicates that the CDF-N stars are the most magnetically active old disk stars. A substantial decline in X-ray luminosities over the 1<t<11 Gyr age interval is required. This is the first demonstration that the coronal and flaring components of stellar magnetic activity -- and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface -- exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types and X-ray luminosities of the sample has Lx ~ 1/t^2 erg/s which is faster than the 1/t decay rate predicted from widely accepted rotational spindown rates and X-ray-activity relations.

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arXiv2004

Chandra observation of an unusually long and intense X-ray flare from a young solar-like star in M78

LkHA312 has been observed serendipitously with the ACIS-I detector on board Chandra with 26h continuous exposure. This H_alpha emission line star belongs to the star-forming region M78 (NGC2068). From the optical and NIR data, we show that it is a pre-main sequence (PMS) low-mass star with a weak NIR excess. This genuine T Tauri star displayed an X-ray flare with an unusual long rise phase (~8h). The X-ray emission was nearly constant during the first 18h of the observation, and then increased by a factor of 13 during a fast rise phase (~2h), and reached a factor of 16 above the quiescent X-ray level at the end of a gradual phase (~6h) showing a slower rise. To our knowledge this flare, with \~0.4-~0.5 cts/s, has the highest count rate observed so far with Chandra from a PMS low-mass star. By chance, the source position, 8.2' off-axis, protected this observation from pile-up. We make a spectral analysis of the X-ray emission versus time, showing that the plasma temperature of the quiescent phase and the flare peak reaches 29MK and 88MK, respectively. The quiescent and flare luminosities in the energy range 0.5--8keV corrected from absorption (N_H~1.7E21 cm^{-2}) are 6E30erg/s and ~1E32erg/s, respectively. The ratio of the quiescent X-ray luminosity on the LkHA312 bolometric luminosity is very high with log(L_X/L_bol)= -2.9, implying that the corona of LkHA312 reached the `saturation' level. The X-ray luminosity of the flare peak reaches ~2% of the stellar bolometric luminosity. The different phases of this flare are finally discussed in the framework of solar flares, which leads to the magnetic loop height from 3.1E10 to 1E11 cm (0.2-0.5 R*, i.e., 0.5-1.3 R_sun).

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arXiv2004

Statistical Challenges in Modern Astronomy

Despite centuries of close association, statistics and astronomy are surprisingly distant today. Most observational astronomical research relies on an inadequate toolbox of methodological tools. Yet the needs are substantial: astronomy encounters sophisticated problems involving sampling theory, survival analysis, multivariate classification and analysis, time series analysis, wavelet analysis, spatial point processes, nonlinear regression, bootstrap resampling and model selection. We review the recent resurgence of astrostatistical research, and outline new challenges raised by the emerging Virtual Observatory. Our essay ends with a list of research challenges and infrastructure for astrostatistics in the coming decade.

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arXiv2003

A Deep Chandra Catalog of X-ray Point Sources toward the Galactic Center

(abridged) We present a catalog of 2357 point sources detected during 590 ks of Chandra observations of the 17-by-17 arcminute field around Sgr A*. This field encompasses a physical area of 40 by 40 pc at a distance of 8 kpc. The completeness limit of the sample at the Galactic center is 10^{31} erg s^{-1} (2.0--8.0 keV), while the detection limit is an order of magnitude lower. The 281 sources detected below 1.5 keV are mainly in the foreground of the Galactic center, while comparisons to the Chandra deep fields at high Galactic latitudes suggest that only about 100 of the observed sources are background AGN. The surface density of absorbed sources (not detected below 1.5 keV) falls off as 1/theta away from Sgr A*, in agreement with the distribution of stars in infrared surveys. Point sources brighter than our completeness limit produce 10% of the flux previously attributed to diffuse emission. The log(N)-log(S) distribution of the Galactic center sources is extremely steep (power-law slope alpha = 1.7). If this distribution extends down to a flux of 10^{-17} erg cm^{-1} s^{-1} (10^{29} erg s^{-1} at 8 kpc, 2.0--8.0 keV) with the same slope, then point sources would account for all of the previously reported diffuse emission. Therefore, the 2.0--8.0 keV luminosity distribution must flatten between 10^{29} - 10^{31} erg s^{-1}. Finally, the spectra of more than half of the Galactic center sources are very hard, and can be described by a power law ($E^{-Gamma}) with photon index Gamma < 1. Such hard spectra have been seen previously only from magnetically accreting white dwarfs and wind-accreting neutron stars, suggesting that there are large numbers of these systems in our field.

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arXiv2003

Ten Million Degree Gas in M 17 and the Rosette Nebula: X-ray Flows in Galactic H II Regions

We present the first high-spatial-resolution X-ray images of two high-mass star forming regions, the Omega Nebula (M 17) and the Rosette Nebula (NGC 2237--2246), obtained with the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer (ACIS) instrument. The massive clusters powering these H II regions are resolved at the arcsecond level into >900 (M 17) and >300 (Rosette) stellar sources similar to those seen in closer young stellar clusters. However, we also detect soft diffuse X-ray emission on parsec scales that is spatially and spectrally distinct from the point source population. The diffuse emission has luminosity L_x ~ 3.4e33 ergs/s in M~17 with plasma energy components at kT ~0.13 and ~0.6 keV (1.5 and 7 MK), while in Rosette it has L_x \~6e32 ergs/s with plasma energy components at kT ~0.06 and ~0.8 keV (0.7 and 9 MK). This extended emission most likely arises from the fast O-star winds thermalized either by wind-wind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse X-ray plasma; in these blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium. These data provide compelling observational evidence that strong wind shocks are present in H II regions.

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arXiv2002

X-rays from star-forming regions: New objects, new physics, new visions

Star-forming regions have been the targets of X-ray observations since the dawn of satellite X-ray astronomy. The increase in sensitivity and/or spatial resolution offered by XMM-Newton and Chandra allows a dramatic improvement, both qualitative and quantitative, on our knowledge of high-energy phenomena in these regions and the underlying physical processes. We summarize here some recent developments: the Orion Nebula Cluster and its 1000+ stellar X-ray sources; Herbig-Haro objects and their high-speed shocks; protostars, brown dwarfs and their unusual magnetic activity; and the discovery of diffuse X-ray emission from HII regions, presumably related to strong winds from massive stars. The role that future X-ray missions may play in the field is already starting to be visible.

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