Discovery of X-Ray Emission From Young Suns in the Small Magellanic Cloud

Abstract

We report the discovery of extended X-ray emission within the young star cluster NGC 602a in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the H ii region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low-and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the new-born stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low-and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.

Photometric Determination of the Mass Accretion Rates of Pre-Main-Sequence Stars. Iv. Recent Star Formation in NGC 602

“…An independent sign of the young ages of the PMS stars identified by the cyan contour lines in Figure 8 comes from X-ray observations. The distribution of our young PMS stars agrees rather well with the extended X-ray emission measured by Oskinova et al (2013) in this field using Chandra. For ease of comparison, we show in Figure 9 the areas of extended Xray emission by means of the contour levels (orange lines) obtained by Oskinova et al (2013), together with an intensity FIG.…”

“…The distribution of our young PMS stars agrees rather well with the extended X-ray emission measured by Oskinova et al (2013) in this field using Chandra. For ease of comparison, we show in Figure 9 the areas of extended Xray emission by means of the contour levels (orange lines) obtained by Oskinova et al (2013), together with an intensity FIG. 9.-Same as Figure 8, but the underlying Hα image is shown in negative, on a grey scale.…”

“…The cyan contours correspond to the Hα luminosity of PMS stars younger than 5 Myr, with the lowest level set to L(Hα) = 10 −5 L and a constant logarithmic step of factor of 3. The orange contour lines are taken from Oskinova et al (2013) and correspond to levels ranging from 5 × 10 −4 cnt s −1 arcmin −2 to 8 × 10 −4 cnt s −1 arcmin −2 on a square-root scale. The dashed lines define the Chandra field of view, while the prominent X-ray source near the top left corner is a background galaxy.…”

Wolf-Rayet stars in the Small Magellanic Cloud

“…Interestingly, AB 8 is the only WR binary in our sample not detected in X-rays. The star was in the field of view of Chandra and XMM-Newton observations (Oskinova et al 2013), but only an upper limit of 5 × 10 32 erg s −1 could be established for its X-ray luminosity. Given the very strong wind of the primary, it is difficult to explain such a low X-ray luminosity; Zhekov & Skinner (2015) recently reported that the X-ray luminosity of the Galactic WO binary WR 30a is >10 The terminal velocity of the companion is found to be comparable to that of the primary.…”

Long-term evolution of the neutron-star spin period of SXP 1062

“…The BeXRB binary SXP 1062 was discovered by Hénault-Brunet et al (2012) using Chandra and XMM-Newton observations of the star-forming region NGC 602 (Oskinova et al 2013b). Amongst other BeXRBs in the SMC, SXP 1062 is outstanding owing to: (i) The position in the wing of the SMC, where a younger stellar population is found and less BeXRBs are known compared to the bar of the SMC.…”