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5 Optical and infrared photometry


   
Table 5: Photometric data for new T Tauri stars in CrA. We list object number as in Tables 2 to 4. Values in brackets are errors for the last digit(s). When no error is given, data are taken from Simbad, GSC, or USNO, which are more uncertain (indicated by colons). For TTS No. 64, the JHK data are from the 2nd 2MASS data release. Bolometric luminosity are estimated for a distance of 130 pc (65 pc for TTS no. 82). Ages and masses are estimated by comparison with isochrones and tracks from D'Antona & Mazzitelli (1994). The last object, RXJ1855.1-3754, is the TTS found in Neuhäuser et al. (1997), a ROSAT HRI source, not a RASS source, see Table 1
No. B V V-R R-I K J-K H-K AV $\log$ age mass
Table 2 mag mag mag mag mag mag mag mag $L_{\rm bol}/L_{\odot}$ $[{\rm Myr}]$ $[M_{\odot}]$
8 14.3: 13.22 (3) 0.98 (1) 1.08 (1) 9.02 (5) 0.86 (6) 0.20 (6) 0.7 -0.45 3.2 0.6
13 12.0: 10.81 (1) 0.50 (2) 0.50 (1) 8.51 (2) 0.62 (3) 0.14 (3) 0.2 0.04 10 1.3
19 18.8: 14.99 (3) 1.24 (3) 1.34 (2) 10.14 (4) 0.91 (16) 0.25 (7) 0.0 -0.68 1.6 0.3
23   9.7:     8.04 (4) 0.41 (15) 0.07 (7) 0.0 0.30 10 1.4
27 12.3: 12.28 (3) 0.70 (1) 0.71 (1) 8.23 (3) 1.35 (5) 0.51 (5) 1.5 -0.04 10.0 1.2
30   11.3:     8.38 (3) 0.71 (5) 0.18 (5) 0.1 0.07 1.0 0.9
31 14.3: 13.1:     9.38 (3) 0.85 (6) 0.22 (5) 0.1 -0.64 6.3 0.6
36 10.0: 9.2:     7.18 (3) 0.44 (5) 0.10 (5) 0.3 0.73 4.0 1.5
39-NE   11.16 (3) 0.73 (2) 0.66 (1) 7.89 (3) 0.72 (5) 0.11 (5) 0.4 0.25 0.5 0.7
39-SW   11.43 (2) 0.24 (1) 0.66 (1) 7.89 (3) 0.72 (5) 0.11 (5) 0.5 0.20 0.5 0.6
53 12.7: 12.35 (3) 0.73 (1) 0.72 (1) 9.01 (3) 0.78 (5) 0.21 (5) 1.0 -0.25 10 1.1
55 11.0: 9.6:     7.31 (3) 0.61 (5) 0.12 (5) 0.1 0.46 2.0 1.7
64 14.9: 12.9:     9.91 (3) 0.91 (5) 0.30 (7) 0.0 -0.25 0.8 0.4
68-E   15.56 (3) 1.2 (1) 1.5 (1) 10.53 (2) 0.96 (4) 0.20 (3) 0.0 -0.91 2.5 0.2
68-W   16.5 (3) 1.2 (3) 1.7 (2) 11.28 (3) 0.95 (5) 0.31 (4) 0.0 -1.29 2.5 0.1
80   15.85 (4) 1.29 (3) 1.52 (2)       3.0 -0.86 40 0.7
82 8.7: 8.2:     7.07 (3) 0.23 (5) 0.05 (5) 0.0 0.30 30 1.2
85 15.5: 14.27 (3) 1.17 (2) 1.23 (2)       1.1 -0.70 7.9 0.6
135 10.8: 9.9:     7.54 (3) 0.69 (5) 0.14 (5) 0.1 0.36 3.2 1.6
RXJ1855.1-3754 13.05 (5) 0.62 (3) 0.57 (3)       0.3 -0.82 40 0.7

Broad band photometric observations in the VRI filters were carried out in 1997 from June 22 to 30 using the 0.9 m telescope of Cerro Tololo Interamerican Observatory (CTIO). The detector was a CCD Tektronix 2048 (CTIO # 3) with pixel size 24 $\mu$m and a readout noise of 3 to 5 electrons. The whole CCD was read out. Dome flat-field exposure sequences in each filter were taken typically before the beginning and upon the end of every night for flat field correction. Two or three different standard star fields from Landolt (1992) were also observed every night at different airmasses for the determination of atmospheric extinction, zero points and color transformation to the Johnson-Kron-Cousins standard system.

Raw CCD frames were bias subtracted and flat fielded using the IRAF package CCDRED. Sky flats were used, and the flat field variation across the final images was spot-checked and found to be negligible. The photometric solution for each night was determined using the IRAF tasks APPHOT and PHOTCAL. First, an aperture for each night was chosen by inspecting the reduced images from each night. The average FWHM over the run was about 1.3 $^{\prime \prime}$. To insure that all the light from a given star was in the aperture, a typical aperture for source extraction was taken as four times the FWHM or about 6 $^{\prime \prime}$. Annuli for background subtraction varied but were usually about 2 $^{\prime \prime}$ from inner diameter to outer diameter with an inner diameter of about 8 $^{\prime \prime}$. The background was taken as the median value within the annuli. An average of 30 calibration standards were observed each night at various airmasses. Photometric solutions for each night were determined by fitting the data to a color dependent airmass equation. The photometric errors dominate the Poisson errors in most cases.

Once the photometric solution for each night was determined, the target fields were examined. Aperture photometry was performed on all stars using the same aperture as was used for the standard stars on that night. The only exception to this was RXJ1857.5-3732. In this case, the two stars were separated by less than 4 $^{\prime \prime}$. Therefore we used the IRAF tool SUBSTAR to subtract one star from the image so we could accurately measure the instrumental magnitude of the other. The observed magnitude and colors were obtained by applying the instrumental magnitudes derived from the aperture to the photometric equation for the given night. Results are listed in Table 5.

The near-IR data were obtained using the CTIO Infra-Red Imager, CIRIM[*], on the 1.5 m telescope at CTIO. CIRIM is a 256 $\times $ 256 HgCdTe array. We observed at the f/13.5 focus, giving a plate scale of 0.65 arcsec per pixel. The data were obtained on the nights of 1998 July 8 to 14. All nights were photometric. The stellar point-spread function is well-sampled photometrically. Dome flats were taken at the start of each night. Flats were obtained with the dome illumination lamps both on and off, to determine the thermal contribution to the flat image. Standard stars, taken from the lists of Elias (1982) and the UKIRT faint standards (Casali & Hawarden 1992), were observed hourly, at a full range of airmasses. Exposure times varied between 0.4 and 3 seconds, based on source magnitude, with 3 images coadded at each position. We observed the standard stars using a 2 $\times $ 2 raster, with 30 arcsec spatial offsets between positions. We observed the targets using exposure times between 0.4 and 20 seconds, based on expected source brightness, with 3 images coadded at each position. We observed using a 2 $\times $ 3 raster, with spatial offsets of 15 arcsec between frames.

We linearized the data using the IRAF routine IRLINCOR. All other processing was undertaken using our IDL-based CIRIM reduction package[*]. The images are divided by the appropriate normalized flat field image. The images in the raster pattern are median-filtered to determine the local sky image, which is then subtracted from each flattened image. The images in the raster pattern are then aligned by cross-correlating on the brightest sources in the image, and co-added. The image center and plate scale are determined by cross-correlating the images with stars in the USNO catalog (Monet et al. 1998). The photometric solution is determined by fitting the log of the standard star counts within a 12-pixel (7.8 arcsec) radius region as a linear function of the air mass. The solution is edited interactively to remove discrepant stars and/or points. The rms scatter in the photometric solutions is 1 to $2\%$, and we take this to be our photometric precision. Magnitudes are determined by applying the photometric solution to the net counts observed in a 12-pixel (7.8 arcsec) radius region. In the case of close binaries, we determined the total flux using a large extraction radius, and the relative magnitudes using smaller extraction radii. Typical uncertainties are $\pm 0.02$ mag, but this degrades for the fainter targets. Results are listed in Table 5.


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