Chandra X-ray Observations of the Young Stellar Cluster NGC 6193 in the Ara OB1 Association

S.L. Skinner (1), S.A. Zhekov(2,3), F. Palla(4) and C.L.D.R. Barbosa(5)

(1) CASA, 389 UCB, Univ. of Colorado, Boulder, CO 80309-0389 USA
(2) JILA, 440 UCB, Univ. of Colorado, Boulder, CO 80309-0389 USA
(3) On leave from Space Research Inst., Moskovska str. 6, Sofia-1000,
(4) INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5,
50125 Firenze, Italy
(5) Instituto de Astronomia, Geofisica e Ciencias Atmosfericas,
Universidade de Sao Paulo, Rua do Matao 1226,
05508-900 Sao Paulo, SP, Brazil

A 90 ksec Chandra HETG observation of the young stellar cluster
NGC 6193 in the southern Ara OB1 association detected 43 X-ray
sources in a 2$'$ $\times$ 2$'$ core region centered on the massive O stars
HD 150135 (O6.5V) and HD 150136 (O3 $+$ O6V). The cluster is dominated
by exceptionally bright X-ray emission from the two O stars, which are
separated by only 10$''$. The X-ray luminosity of HD 150136 is
log L$_{\rm X}$ = 33.39 (ergs s$^{-1}$), making it one of the most
luminous O-star X-ray sources known. All of the fainter X-ray sources
in the core region have near-IR counterparts, but existing JHK photometry
provides little evidence for near-IR excesses. These core sources have
typical mean photon energies $\langle$E$\rangle$ $\approx$ 2 keV and
about one-third are variable. It is likely that some are young low-mass
stars in the cluster, but cluster membership remains to be determined.
Grating spectra show
that the X-ray properties of HD 150135 and HD 150136 are similar,
but not identical. Both have moderately broadened unshifted emission
lines and their emission is dominated by cool plasma at kT $\approx$ 0.3 keV,
pointing to a wind-shock origin. However, the emission of HD 150136 is
slightly hotter and four times more luminous than its optical twin
HD 150135. We discuss the possibility that a radiative colliding wind
shock contributes to the prodigious X-ray output of the short-period
(2.66 d) spectroscopic binary HD 150136. A suprising result
is that the X-ray emission of HD 150136 is slowly variable on a timescale
of $<$1 day. The origin of the variability is not yet known but the
observed behavior suggests that it is an occultation effect.

Reference: MNRAS, in press
Status: Manuscript has been accepted