F. R. Harnden

STELLAR ACTIVITY ON THE YOUNG SUNS OF ORION: COUP OBSERVATIONS OF K5-7 PRE-MAIN-SEQUENCE STARS

In 2003 January, the Chandra Orion Ultradeep Project (COUP) detected about 1400 young stars during a 13.2 day observation of the Orion Nebula Cluster (ONC). This paper is a study of the X-ray properties of a well-defined sample of 28 solar-mass ONC stars based on COUP data. Our goals are to characterize the magnetic activity of analogs of the young Sun and thereby to improve understanding of the effects of solar X-rays on the solar nebula during the era of planet formation. Given the length of the COUP observation we are able to clearly distinguish characteristic and flare periods for all stars. We find that active young suns spend 70% of their time in a characteristic state with relatively constant flux and magnetically confined plasma with temperatures kT2 2.1 × kT1. During characteristic periods, the 0.5-8 keV X-ray luminosity is about 0.03% of the bolometric luminosity. One or two powerful flares per week with peak luminosities log LX ~ 30-32 ergs s-1 are typically superposed on this characteristic emission accompanied by heating of the hot plasma component from 2.4 to 7 keV at the flare peak. The energy distribution of flares superposed on the characteristic emission level follows the relationship dN/dE ∝ E-1.7. The flare rates are consistent with the production of sufficiently energetic protons to spawn a spallogenic origin of some important short-lived radionuclides found in ancient meteorites. The X-rays can ionize gas in the circumstellar disk at a rate of 6 × 10-9 ionizations per second at 1 AU from the central star, orders of magnitude above cosmic-ray ionization rates. The estimated energetic particle fluences are sufficient to account for many isotopic anomalies observed in meteoritic inclusions.

Chandra X-Ray Observation of the Orion Nebula Cluster. II. Relationship between X-Ray Activity Indicators and Stellar Parameters

Using the results of our first paper on the Chandra HRC observation of the Orion Nebula cluster (ONC), we explore the relation between the coronal activity of its 1 Myr-old pre-main-sequence population and stellar parameters. We find that median X-ray luminosities of low-mass stars (M/M☉ 3) increase with increasing mass and decreasing stellar age. Brown dwarfs (0.03 M/M☉ 0.08) follow the same trend with mass. From M ~ 0.1 to M ~ 0.5 M☉, median LX/Lbol values increase by about half an order of magnitude and then remain constant at ~10-3.5 for the mass range from 0.5 to 3.0 M/M☉. In these same two mass ranges, LX/Lbol remains roughly constant with age, until it drops by more than 2 orders of magnitudes at the epoch when ~2-4 M☉ stars are expected to become fully radiative. We find a dependence of LX and LX/Lbol on circumstellar accretion indicators and suggest three possible hypotheses for its origin. In spite of improved X-ray and rotational data, correlations between activity indicators and rotation remain elusive for these stars, possibly indicating that stars for which rotational periods have been measured have reached some saturation level. Our study of X-ray activity versus stellar mass leads us to propose that the few HRC X-ray sources not associated with any optical/infrared counterpart trace a yet-to-be-discovered stellar population of deeply embedded, relatively massive ONC members.

Chandra X-Ray Observation of the Orion Nebula Cluster. I. Detection, Identification, and Determination of X-Ray Luminosities

In this first of two companion papers on the Orion Nebula Cluster (ONC), we present our analysis of a 63 ks Chandra HRC-I observation that yielded 742 X-ray detections within the 30 × 30 field of view. To facilitate our interpretation of the X-ray image, here we collect a multiwavelength catalog of nearly 2900 known objects in the region by combining 17 different catalogs from the recent literature. We define two reference groups: an infrared sample, containing all objects detected in the K band, and an optical sample comprising low-extinction, well-characterized ONC members. We show for both samples that field object contamination is generally low. Our X-ray sources are primarily low-mass ONC members. The detection rate for optical sample stars increases monotonically with stellar mass from zero at the brown dwarf limit to ~100% for the most massive stars but shows a pronounced dip between 2 and 10 M☉. We determine LX and LX/Lbol for all stars in our optical sample and utilize this information in our companion paper to study correlations between X-ray activity and other stellar parameters.

An Einstein Observatory SAO-based catalog of B-type stars

About 4000 X-ray images obtained with the Einstein Observatory are used to measure the 0.16-4.0 keV emission from 1545 B-type SAO stars falling in the about 10 percent of the sky surveyed with the IPC. Seventy-four detected X-ray sources with B-type stars are identified, and it is estimated that no more than 15 can be misidentified. Upper limits to the X-ray emission of the remaining stars are presented. In addition to summarizing the X-ray measurements and giving other relevant optical data, the present extensive catalog discusses the reduction process and analyzes selection effects associated with both SAO catalog completeness and IPC target selection procedures. It is concluded that X-ray emission, at the level of Lx not less than 10 exp 30 ergs/s, is quite common in B stars of early spectral types (B0-B3), regardless of luminosity class, but that emission, at the same level, becomes less common, or nonexistent, in later B-type stars.

Chandra Observations of the Open Cluster NGC 2516

Our analysis of Chandra X-Ray Observatory data for the open cluster NGC 2516, sometimes referred to as the southern Pleiades, has yielded over 150 X-ray detections in both High-Resolution Camera and Advanced CCD Imaging Spectrometer images of the central region of the cluster. We identify some of the new X-ray sources with photometric cluster members and compare these new Chandra results with those of ROSAT. To date, 82 detected X-ray sources (42% of surveyed cluster members) are tentatively identified as cluster members. We also discuss the X-ray properties of late-type members in comparison with those of corresponding stellar types in the more metal-rich, approximately coeval Pleiades Cluster.

Coronal Variability in the Young Cluster NGC 2516

NGC 2516 has been observed by Chandra several times in order to correct the plate scale of the spacecrafts focal plane instruments. Because of this, Chandra has observed NGC 2516 with all four imaging arrangements available. In addition, NGC 2516 has been observed as part of the High Resolution Camera (HRC) guaranteed time program and is scheduled for return plate scale calibration visits. This makes it the best cluster to study for long-term variability. NGC 2516 is about 140 Myr old and less than 400 pc away. In our first paper, we discussed the detection of 150 X-ray sources (42% of which are identified as cluster members) in the calibration data taken during the orbital activation phase of the Chandra mission. In our second paper, we combined all the extant data sets and detected 284 sources, more than half of which are considered likely cluster members. In this our third paper, we further explore techniques of combining Advanced CCD Imaging Spectrometer (ACIS) and HRC Chandra data for timing analysis. We have been able to combine almost 70 ks of observation time, spread over five epochs, to study variability in this cluster on multiple timescales. We find that while stochastic variability rates are about the same for all objects in the sample, the timescale for detecting variability is shorter for late-type stars. Both stochastic and flare variability rates seen in NGC 2516 are similar to those seen in younger clusters IC 348, NGC 1333, and M42.