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Description
Empirical calibrations of the stellar age-rotation-activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. The authors used the Chandra X-ray Observatory to study M 37 (NGC 2099), a 500-Myr-old open cluster that has been extensively surveyed for rotation periods (P<sub>rot</sub>). M 37 was observed almost continuously for five days, for a total of 440.5 ks, to measure stellar X-ray luminosities (L<sub>X</sub>), a proxy for coronal activity, across a wide range of masses. The cluster's membership catalog was revisited to calculate updated membership probabilities from photometric data and each star's distance to the cluster center. The result is a comprehensive sample of 1699 M 37 members: 426 with P<sub>rot</sub>, 278 with X-ray detections, and 76 with both. The authors calculate Rossby numbers, R<sub>o</sub>= P<sub>rot</sub>/tau , where tau is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, L<sub>X</sub>/L<sub>bol</sub>, to minimize mass dependencies in their characterization of the rotation-coronal activity relation at 500 Myr. They find that fast rotators, for which R<sub>o</sub> < 0.09 +/- 0.01, show saturated levels of activity, with log(L<sub>X</sub>/L<sub>bol</sub>) = -3.06 +/- 0.04. For R<sub>o</sub> >= 0.09 +/- 0.01, activity is unsaturated and follows a power law of the form R<sup>beta</sup><sub>o</sub> , where beta = -2.03 (-0.14, +0.17). This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4 - 1.3 solar masses, P<sub>rot</sub> in the range 0.4 - 12.8 days, and L<sub>X</sub> in the range 10<sup>28.4</sup> - 10<sup>30.5</sup> erg s<sup>-1</sup>. These results make M 37 a new benchmark open cluster for calibrating the ARAR at ages of ~ 500 Myr. The central field of M 37 was observed five separate times between 2011 November 14 20:58 and 2011 November 1915:31 UTC for a total of 440.5 ks with the Advanced CCD Imaging Spectrometer (ACIS). The four ACIS-I chips and the ACIS-S3 chip were used in Very Faint telemetry mode to improve the screening of background events and thus increase the sensitivity of ACIS to faint sources. The exposure-weighted average aimpoint of the 16.9 x 16.9 arcmin<sup>2</sup> ACIS-I field of view is RA = 05<sup>h</sup> 52<sup>m</sup> 17.86<sup>s</sup>,Dec = +32<sup>o</sup> 33' 48.23" (J2000). The pitch angle for four observations was 103 degrees; due to scheduling constraints, it was 253 degrees for the fifth. Table 1 in the reference paper provides the basic information for the 5 Chandra observations and Figure 1 in that paper shows their footprints superimposed on a 40' x 40' i' image centered on M 37 obtained by Hartman et al. (2008, ApJ, 675, 1233, hereafter HA08) with the Megacam on the MMT telescope. This HEASARC table contains all of the data from Table 3 of the reference paper, the M 37 Chandra catalog of 774 X-ray sources, and the data from Table 5, the catalog of optical objects, for those objects which have been identified as optical counterparts to the X-ray sources. It does not contain entries for those optical objects in Table 5 which lack X-ray counterparts. This online catalog was created by the HEASARC in December 2015 based on machine-readable versions of tables 3 and 5 from the paper which were obtained from the ApJ website. This is a service provided by NASA HEASARC .
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