The observed photometric RMS in the time series data as a function of magnitude for stars brighter than the defined completeness limits. The observed RMS ranges from about 0.020 mag for the brightest stars to <= 0.15 mag (i.e. signal to noise ratio >= 7) for stars at the completeness limit. The scale on the y-axis is the same for each panel.
The Stetson variable index (S) plotted as a function of the H magnitude for stars brighter than H=15.2. The dashed line at S=0 shows the expected value of the variability index for non-variable stars. The origin of the positive bias in the computed index values is unknown, and suggests that a weak correlation exists between the J, H, and K_s photometry, possibly from the fact that the three bands were observed at the same time. The dotted line at S=1.00 represents the minimum adopted value used to identify variable stars in this study. Note that 7 stars with S > 5.0 are not shown.
Photometric data for star 11344 (also known as T 29 and Sz 22) that illustrates the data obtained for this study. The left and middle panels show the J, H, K_s, J-H and H-K_s light curves. The vertical bars through the data points represent the ± 1 sigma photometric uncertainties. The right panels show the K_s vs. H-K color-magnitude diagram and the J-H vs. H-K color-color diagrams for each data point in the time series, where the dotted line represents the interstellar reddening vector from Cohen et al. (1981) transformed into the 2MASS photometric system (Carpenter 2001). The uncertainties in the stellar colors have been omitted for clarity. The solid line in the color-magnitude diagram is the 2 Myr pre-main-sequence isochrone from D'Antona & Mazzitelli (1997,98) for stellar masses between 0.08 Mo and 3 Mo. The solid curves in the color-color diagram are the loci of red giant and main-sequence stars from Bessell & Brett (1988) in the 2MASS color system.
Spatial distribution of stars toward the Chamaeleon I molecular cloud. Starting with the leftmost panel, these figures show
- the spatial distribution of sources with J <= 16;
- location of variable stars identified from our near-infrared data, where filled symbols indicate variable stars that were previously identified as Chamaeleon I members;
- sources that exhibit a near-infrared excess in the J-H vs. H-K_s color-color diagram, where filled triangles represent variable stars, and open triangles indicate non-variables brighter than K_s=13.5;
- members and candidate members of the Chamaeleon I molecular cloud identified prior to this study (see Appendix);
- x-ray sources selected from the ROSAT All Sky Survey, where filled squares represent x-ray sources that have been associated with pre-main-sequence objects and open squares represent objects unrelated to Chamaeleon I (Alcala et al. 1995); and
- a map of the average H-K_s stellar color with 5' resolution, where the contour levels are at 0.20, 0.35, 0.50 mag, and increments of 0.30 mag thereafter.
K_s vs. H-K_s color-magnitude diagram for all stars (color scale) and the variable stars (circles). Filled circles indicate stars which have been previously identified as members of Chamaeleon I. The solid black curve shows the 2 Myr pre-main-sequence isochrone from D'Antona & Mazzitelli (1997,98) for masses between 0.017 Mo and 3.0 Mo, and the green curve shows the 10 Myr isochrone. The dashed lines indicate the reddening vector for 10 magnitudes of visual extinction from Cohen et al. (1981) transformed into the 2MASS photometric system (Carpenter 2001), where the reddening vector is drawn at 0.017, 0.08, and 3.0 Mo. This figure shows that a large number of the variable stars are consistent with reddened pre-main-sequence stars with masses < 3 Mo. A second group of variable stars are faint and blue, and as discussed in the text, are most likely variable field stars or old pre-main-sequence stars unrelated to Chamaeleon I.
J-H vs. H-K color-color diagram for all stars (color scale) and the 95 variable stars (circles) identified from the J, H, and K_s time-series data. The filled circles represent variable stars that were previously identified as likely Chamaeleon I members.
Spatial distribution of variable stars for two different color ranges. The left panel shows the distribution of relatively blue variables with H-K_s < 0.3, and the right panel shows the distribution of red variables with H-K_s > 0.3. The blue variable stars are found over the entire region, while the red variable stars, not surprisingly, are located mainly toward the molecular cloud. Of the 4 red variable stars found well outside the cloud boundaries, one is thought to be a Mira variable, one is a known pre-main-sequence star identified from the ROSAT All Sky Survey (Alcala et al. 1995), and the remaining 2 are of unknown origin. As discussed in the text, the faint, blue variable stars are most likely field stars or an older population (> 10 Myr) of low mass pre-main-sequence stars located beyond the Chamaeleon I cloud.
