Evidence for Variable, Correlated X-ray and Optical/IR Extinction toward the Nearby, Pre-main Sequence Binary TWA 30

Abstract

We present contemporaneous XMM-Newton X-ray and ground-based optical/near-IR spectroscopic observations of the nearby ($D\approx 42$ pc), low-mass (mid-M) binary system TWA 30A and 30B. The components of this wide (separation $\sim$3400 AU) binary are notable for their nearly edge-on disk viewing geometries, high levels of variability, and evidence for collimated stellar outflows. We obtained XMM-Newton X-ray observations of TWA 30A and 30B in 2011 June and July, accompanied (respectively) by IRTF SpeX (near-IR) and VLT XSHOOTER (visible/near-IR) spectroscopy obtained within $\sim$20 hours of the X-ray observations. TWA 30A was detected in both XMM-Newton observations at relatively faint intrinsic X-ray luminosities ($L_X$$\sim$$8\times 10^{27}$ $erg$ $s^{-1}$) compared to stars of similar mass and age . The intrinsic (0.15-2.0 keV) X-ray luminosities measured in 2011 had decreased by a factor 20-100 relative to a 1990 (ROSAT) X-ray detection. TWA 30B was not detected, and we infer an upper limit of ($L_X$ $\lesssim$ 3.0 $\times$ $10^{27}$ erg s${}^{-1}$). We measured a large change in visual extinction toward TWA 30A (from $A_V\approx 14.9$ to $A_V\approx 4.7$) between the two 2011 observing epochs, and we find evidence for a corresponding significant decrease in X-ray absorbing column ($N_H$). The apparent correlated change in $A_V$ and $N_H$ is suggestive of variable obscuration of the stellar photosphere by disk material composed of both gas and dust. However, in both observations, the inferred $N_H$ to $A_V$ ratio is lower than that typical of the ISM, suggesting that the disk is either depleted of gas or is deficient in metals in the gas phase.

arXiv comment10 pages, 7 figures, Accepted for publication in MNRAS

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