Deep ROSAT HRI Observations of the Orion Nebula Region
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 1995
- DOI:
- 10.1086/175694
- Bibcode:
- 1995ApJ...445..280G
- Keywords:
-
- 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;
- Astronomy;
- ISM: INDIVIDUAL NAME: ORION NEBULA;
- STARS: PRE--MAIN-SEQUENCE;
- X-RAYS: STARS