Near-Infrared Photometric Variability of Stars
Toward the Chamaeleon I Molecular Cloud


John M. Carpenter
(jmc@astro.caltech.edu)

Lynne A. Hillenbrand
(lah@astro.caltech.edu)

M. F. Skrutskie
(skrutskie@virginia.edu)

and

Michael R. Meyer
(mmeyer@as.arizona.edu)



Abstract

We present the results of a J, H, and K_s photometric monitoring campaign of a 0.72 x 6 sq. deg. area centered on the Chamaeleon I star forming region. Data were obtained on 15 separate nights over a 4 month time interval using the 2MASS South telescope. Out of a total of 34,539 sources brighter than the photometric completeness limits (J=16.0, H=15.2, K_s=14.8), 95 exhibit near-infrared variability in one or more bands. The variables can be grouped into a population of bright, red objects that are associated with the Chamaeleon I association, and a population of faint, blue variables that are dispersed over the full 6 deg of the survey and are likely field stars or older pre-main-sequence stars unrelated to the present-day Chamaeleon I molecular cloud. Ten new candidate members of Chamaeleon I, including 8 brown dwarf candidates, have been identified based on variability and/or near-infrared excess emission in the J-H vs. H-K_s color-color-diagram. We also provide a compendium of astrometry and J, H, and K_s photometry for previously identified members and candidate members of Chamaeleon I.





Figures
(click on thumbnails to see enlarged versions of figures)


Figure 1:
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.


Figure 2:
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.


Figure 3:
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.


Figure 4:
Spatial distribution of stars toward the Chamaeleon I molecular cloud. Starting with the leftmost panel, these figures show
  1. the spatial distribution of sources with J <= 16;
  2. location of variable stars identified from our near-infrared data, where filled symbols indicate variable stars that were previously identified as Chamaeleon I members;
  3. 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;
  4. members and candidate members of the Chamaeleon I molecular cloud identified prior to this study (see Appendix);
  5. 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
  6. 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.
These panels show that the largest concentration of variable stars is toward the Chamaeleon I molecular cloud despite the overall decrease in the stellar surface density, indicating many of the variable stars must be associated with the Chamaeleon I molecular cloud.


Figure 5:
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.


Figure 6:
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.


Figure 7:
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.


Figure 8: J-H vs. H-K_s color-color diagrams in four different K_s-band magnitude intervals for objects in the source list with at least 10 J-H and H-K_s measurements. The number of sources shown in each panel are 2869 for K_s < 12, 10070 for 12 < K_s < 14, 6336 for 14 < K_s < 14.5 and 5320 for 14.5 < K_s < 14.8. Sources with a near-infrared excess and brighter than K_s=14 are most likely stars associated with Chamaeleon I and surrounded by an accretion disk. The fainter sources with an infrared excess are most likely galaxies with redshifts of z < 0.25 as discussed in the text.


Figure 9: K_s vs. H-K_s color-magnitude diagram for all sources (color scale) and sources with a near-infrared excess (triangles) detectable in the J-H vs. H-K_s color-color diagram. Filled triangles represent the subset of sources with excesses that are also variable. Sources with excesses and brighter than K_s=13.5 (see Fig.4) are clustered around the Chamaeleon I molecular cloud and are most likely pre-main-sequence stars (for K_s < 11) and brown dwarf candidates (for 11 < K_s < 13.5). Objects fainter than K_s=13.5 are distributed over the entire survey area and are most likely galaxies as discussed in the text.