Deep ROSAT HRI observations of the Orion nebula region

Details Deep ROSAT HRI observations of the Orion nebula region Deep ROSAT HRI observations of the Orion nebula region

May 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...445..280g&link_type=abstract

The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 445, no. 1, p. 280-313

Astronomy and Astrophysics

Astrophysics

83

Orion Nebula, Pre-Main Sequence Stars, Stellar Flares, Stellar Rotation, X Ray Density Measurement, X Ray Imagery, X Ray Sources, Data Reduction, Flux Density, Image Analysis, Luminosity, Position (Location), Rosat Mission, Spatial Resolution

Scientific paper

We present results from three deep ROSAT high-resolution imager observations of the Orion Nebula star-forming region. The X-ray images contain over 1500 cataloged stars in a roughly 0.8 sq deg region centered on the Trapezium. In all, 389 distinct X-ray sources have been detected, at least two-thirds of which are associated with a single proper-motion cluster member. X-ray emission is detected from stars of all spectral types, from massive O- and B-type components of the Trapezium to the coolest, low-mass pre-main-sequence (PMS) stars. In this paper we focus primarily on X-ray emission from the late-type PMS stars. Of the approximately 100 late-type cluster members with measured spectral types, approximately three-fourths have been detected; we have derived X-ray luminosity upper limits for the remaining stars. We found coronal X-ray emission turns on around spectral type F6, with the upper envelope of activity increasing with deceasing effective temperature. When plotted in an X-ray luminosity versus bolometric luminosity diagram, late-type PMS stars lie below a 'saturation' line corresponding to Lx/Lbol approximately 10-3. For approximately solar-mass PMS stars, we find a median X-ray luminosity approximately 1 x 1030 ergs/s. The late G, K, and M stars exhibit nearly a two order of magnitude spread in X-ray luminosity and in Lx/Lbol at a given effective temperature. Plots of X-ray activity versus v sin i rotational velocity and rotational period appear to show no clear dependence of activity on rotation. However, because only a small fraction of late-type PMS stars in the Orion Nebula have measured v sin i or Prot and because of uncertainties in Lx and Lx/Lbol, we believe the data are not conclusive on this point. Light curves of the detected X-ray sources have revealed at least 10 strong X-ray flares with characteristic rise times greater than or approximately equal to 1 hr and decay times ranging from approximately 2 to 12 hr. All the flares have X-ray energies in excess of 3 x 1035 ergs. Many of the X-ray sources associated with late-type cluster members have been observed in more than one ROSAT HRI exposure, allowing us to look for variability on the approximately 1 yr timescale between HRI observations. A statistical analysis of the resulting count rate ratios suggests that at least one-fourth of the late-type cluster stars are significantly variable on this timescale.

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