We report on new X-ray observations of the classical T Tauri star DG Tau. DG Tau drives a collimated bi-polar jet known to be a source of X-ray emission perhaps driven by internal shocks. The rather modest extinction permits study of the jet system to distances very close to the star itself. Our initial results presented here show that the spatially resolved X-ray jet has been moving and fading during the past six years. In contrast, a stationary, very soft source much closer (~ 0.15-0.2") to the star but apparently also related to the jet has brightened during the same period. We report accurate temperatures and absorption column densities toward this source, which is probably associated with the jet base or the jet collimation region.
DG Tau is a classical T Tauri star (CTTS) with a flat infrared spectrum, indicating the presence of substantial circumstellar material. DG Tau ejects a well-studied bipolar jet, showing several knots and shocks (e.g., Eislöffel & Mundt 1998). The most prominent knots are presently located about 5 ′′ and 12 ′′ away from the star, toward the SW. The NE counter-jet is difficult to see owing to absorption/extinction by the foreground extended disk structure (Kitamura et al. 1996). We discovered DG Tau's jet in X-rays (Güdel et al. 2005(Güdel et al. , 2007(Güdel et al. , 2008;;G05, G07, resp. G08 henceforth) along with a spectral anomaly in the central source also ascribed to the jet. This anomaly manifests itself in a superposition of two unrelated spectral components subject to different hydrogen absorption column densities, N H , in the unresolved central point spread function (PSF) that contains the star itself (defining a "Two-Absorber X-Ray Spectrum" = TAX, G07). The hard component is ascribed to the flaring corona/magnetosphere of the star, excessively absorbed by dust-depleted accretion streams, while the little absorbed soft component is ascribed to X-ray emission from the jet base. We have started a multiwavelength campaign to study DG Tau in detail, centered around a Chandra Large Program (360 ks of ACIS-S time in January 2010, complementing our 90 ks obtained in 2004-06) and involving radio, millimeter, infrared, and optical telescopes. Here, we present initial results, focusing on the X-ray phenomenology.
The DG Tau X-ray jet is visible at PA=225 deg out to a distance of 5-6 ′′ . Figure 1 is based on the January 2010 observations (360 ks). To optimize spatial resolution, we reconstructed events files for the detector’s “VFAINT” mode (allowing for better background treatment), removing standard pixel randomization and applying the subpixel event repositioning (SER) method by Li et al. (2003) and Li et al. (2004). The image has been smoothed using a Gaussian kernel of width 1.2 pixels (1 pix = 0.25 ′′ ), and intensity has been logarithmically compressed, including only 0.4-1.5 keV counts. The jet to the SW appears to be brightest toward its apex. This source coincides with an optical knot (see below). It is therefore likely that the gas has been heated by shocks forming in the jet.
The optical jet expands by 0.15-0.3 ′′ yr -1 (Eislöffel & Mundt 1998;Dougados et al. 2000). The 2-color image in Fig. 2a shows a superposition of the (smoothed) X-ray data from Winter 2005/06 (green, 60 ks) and January 2010 (red). Jet motion is discernible. Using the wavdetect task in CIAO (on unsmoothed data), we find a velocity of ≈ 0.2 ′′ yr -1 toward PA = 225 deg, coincident with the optical velocity. The X-ray shock regions are thus co-moving with the jet and are clearly not standing shocks. The X-ray jet apparently faded between 2004 and 2010. X-ray count rates in identical areas of the SW jet were 0.20±0.08 ct ks -1 (1σ, 2004), 0.18±0.06 ct ks -1 (2006), and 0.11±0.02 ct ks -1 (2010), indicating a marginal trend consistent with cooling estimates in G08 for rather high electron densities (e.g., 10 5 cm -3 ). Figure 2b shows a superposition of X-rays (red) with blueshifted [S ii] (-250 km s -1 to -100 km s -1 , green) observed with PMAS (Potsdam Multi Aperture Spectrophotometer) attached to the 3.5 m telescope at Calar Alto (mean seeing was 1.8 ′′ , pixel size 1 ′′ × 1 ′′ , observing date 16 November 2009). The [S ii] knot is located at the tip of the soft X-ray jet.
The 2004-06 Chandra data (90 ks) showed a clear X-ray detection of the counter jet (Fig. 3, left; 9 counts, see G08). Although still dominated by soft photons, its spectral appearance is harder than the forward jet. This has been ascribed to photoelectric absorption by the foreground extended disk. The excess N H (compared to the forward jet) was found to be 3 × 10 21 cm -2 , compatible with the excess extinction toward the counter jet (G08). The image also shows -marginally -that the forward jet may be harder at its apex; this may be a sign of shock heating and post-shock cooling.
The counter-jet is hardly seen in the 4x longer 2010 exposure (Fig. 3, right) although we had expected to find 36 counts. The cause is unclear; most likely the jet has cooled by expansion and radiation (which is true for the forward jet as well -see below).
The unresolved central point-spread function shows spectral TAX phenomenology, i.e., two unrelated thermal components subject to different absorption. The hard (1.5-7.3 keV) light curves (Fig. 4) and spectra (Fig. 5) show frequent flares, while the soft (0.5-1 keV) component is steady. However, the soft emission gradually increased from 2004 to 2010, corresponding to an increase of the X-ray luminosity from 1.1 × 10 29 erg s -1 (2004), to 1.2 × 10 29 erg s -1 (2006), to 1.8 × 10 29 erg s -1 (2010), while the temperature and absorption did not change.
The soft component indicates unusually low temperatures for X-ray sou
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