Astron. Astrophys. 354, L9-L12 (2000)

3. Infrared imaging

TWA-7 was observed on 26 March 1998 with HST NIC2 (Near-Infrared Camera and Multi-Object Spectrometer, NICMOS) in coronographic mode with F160W (PI E. Becklin, GTO 7226). Between two 224s exposures, HST was rotated by to facilitate the subtraction of instrumental signatures. We retrieved the individual pipeline calibrated images from the archive and subtracted them from each other to remove the diffraction pattern and the scattered light halo around TWA-7 (see Fig. 2).

Fig. 2. Images of TWA-7A and B: HST NICMOS F160W filter ( H-band) where the bright star is partly occulted by the coronograph (left) and our NTT SHARP K-band image (middle), we also detected TWA-7B in H using SHARP. In the right panel, we show the 1 error ellipses of the positions of TWA-7B relative to TWA-7A at the three different epochs: At the first observation (NIC2, 1998.2) TWA-7A is at (0,0), then moved by its known proper motion to the positions when observed with NIC1 (1998.8) and NTT (1999.5); from left to right, with small error circles in the TWA-7A positions due to error in proper motion; we use the STARNET proper motion, but the result is basically the same when using the SPM proper motion. For TWA-7B, the small innermost error ellipse is for NIC1 and the large outermost one for NTT. Due to the large astrometric errors in the NIC2 and NTT observations, TWA-7B could be either a background object or a companion to TWA-7A

A faint object south-east of TWA-7 clearly stands out on the subtracted data (designated 1RXSJ104230.3- 334014B = TWA-7B). Centroiding TWA-7 behind the coronographic mask is problematic, because the mask itself is not symmetric and is shifting its position by up to pixel within one orbit. We fitted and extrapolated the diffraction spikes to obtain their central crossing, which can be taken as the approximate location of the centroid of the occulted star, and obtained a separation of at a position angle of (including for uncertainty in N-S alignment). The F160W magnitude of TWA-7B is .

On 19 June 1999, we observed the pair using SHARP (System for High Angular Resolution Pictures, Hofmann et al. 1992) at the ESO 3.5m New Technology Telescope (NTT) before the main targets of that program (63.N-0178) became visible at low airmass.

The north-south alignment and the pixel scale of the camera were measured using images of the galactic center taken in the same night and precise radio positions of those stars (Menten et al. 1997). We found the orientation to be off by , namely tilted from N to W, and the pixel scale to be arc sec per pixel.

The SHARP speckle images consist of s exposure in H and s in K, see Fig. 2. The short exposure seeing in the near-infrared during that night was better than . The data were corrected for bad pixels followed by a sky image subtraction and the application of a flat-field. For each band we then co-added the pixel frames using the brightest pixel of TWA-7 as shift-and-add reference (Christou 1991). We measure a separation between TWA-7 and 7B of and, after correcting for the misalignment, a position angle of , consistent in both the H- and K-band image.

Using the standard star HR 4013 (Bouchet et al. 1991), observed just after TWA-7, we obtain H=7.11 and K=6.91 mag for TWA-7, within 0.02 mag of Webb et al. (1999). For TWA-7B, we derive H= mag and K= mag, i.e. more than nine magnitudes fainter than TWA-7. These values are in agreement with the NICMOS data, especially considering the systematic offset between the HST Vega system ¹ and ground-based photometric systems for red objects of up to mag.

TWA-7 was again observed with HST NICMOS on 2 Nov 1998 (PI E. Becklin, GTO 7226). We found for TWA-7B the following magnitudes: in F090M (NIC1), in F165M (NIC2), and in F180M (NIC2). From the NIC1 image, the only one taken without coronograph, we obtained and for separation and position angle, respectively, between TWA-7 and 7B, consistent with the other observations.

© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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