Constraints on the Circumstellar Disk Masses in the IC 348 Cluster


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




Abstract

A 5.2' x 5.2' region toward the young cluster IC 348 has been imaged in the millimeter continuum at 4.0" x 4.9" resolution with the OVRO interferometer to a RMS noise level of 0.75 mJy/beam at 98 GHz. The data are used to constrain the circumstellar disk masses in a cluster environment at an age of about 2 Myr. The mosaic encompasses 95 known members of the IC 348 cluster with a stellar mass distribution that peaks at 0.2-0.5 Msun. None of the stars are detected in the millimeter continuum at an intensity level of 3 sigma or greater. The mean observed flux for the ensemble of 95 stars is 0.22 ± 0.08 mJy. Assuming a dust temperature of 20 K, a mass opacity coefficient of kappa_o = 0.02 cm^2/g at 1300 um, and a power law index of beta=1 for the particle emissivity, these observations imply that the 3 sigma upper limit to the disk mass around any individual star is 0.025 Msun, and that the average disk mass is 0.002 +/- 0.001 Msun. The absence of disks with masses in excess of 0.025 Msun in IC 348 is different at the about 3 sigma confidence level from Taurus, where about 14% of the stars in an optically selected sample have such disk masses. Compared with the minimum mass needed to form the planets in our solar system (about 0.01 Msun), the lack of massive disks and the low mean disk mass in IC 348 suggest either that planets more massive than a few Jupiter masses will form infrequently around 0.2-0.5 Msun stars in IC 348, or that the process to form such planets has significantly depleted the disk of small dust grains on time scales less than the cluster age of about 2 Myr.

To appear in the Astronomical Journal (Preprint - gzipped postscript file)



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


Figure 1:
Schematic of the IC 348 mosaic made with OVRO. The solid squares mark the 64 pointing centers used to create the mosaic. The pointing centers are separated by 40" along a row, with adjacent rows also separated by 40". For comparison, the circle in the lower right corner shows the FWHM beam size of a single OVRO antenna (72") at the observed frequency 98 GHz. The dotted curve shows the extent of the OVRO mosaic (~ 5.2' x 5.2') at the unit gain boundary. The synthesized beam size is 4.0" x 4.9".


Figure 2:
Bottom: Histogram of the stellar masses for the IC 348 cluster members located within the OVRO mosaic. Three probable IC 348 members within the OVRO mosaic did not have sufficient data to infer their stellar properties and are not shown in this figure.

Top: Histograms of the stellar masses for a comparison sample of stars in the Taurus molecular cloud that have available optical spectroscopy, photometry, and submillimeter continuum observations. The open histogram represents all stars in the comparison sample, and the hatched histogram shows the stellar mass distribution for sources that have a disk mass greater than the 3sigma detection limit of 0.025 Msun for the IC 348 observations. The stellar masses in both IC 348 and Taurus have been inferred by placing the stars in an HR diagram using the database compiled by Hillenbrand, Meyer, and Carpenter (2002) and D'Antona and Mazzitelli (1997,98) pre-main-sequence evolutionary tracks.



Figure 3:
Bottom: Histogram of the stellar ages for the IC 348 cluster members within the OVRO mosaic. Top: Histograms of the stellar ages for a comparison sample of stars in the Taurus molecular cloud as described in Figure 2.


Figure 4:
Left: J-H vs. H-Ks color-color diagram for 82 IC 348 members in the OVRO mosaic that have photometry in the 2MASS database without any error flags.

Right: J-H vs. Ks-L color-color diagram for 47 IC 348 members in the OVRO mosaic with 2MASS photometry and L-band photometry from Haisch etal. (2001a). In each panel, the solid curves represent the locus of main-sequence and giant stars (Bessell & Brett 1988) and the dashed line is the interstellar reddening vector (Cohen etal. 1981), where the J-H and H-K colors have been transformed into the 2MASS photometric system (Carpenter 2001). Four stars exhibit an apparent H-Ks excess, but in each instance, the magnitude of the excess is less than the 1 sigma photometric uncertainties and can be attributed to photometric noise. In the J-H vs. Ks-L diagram, 18 stars show an apparent Ks-L excess, of which 14 have an excess larger than the estimated photometric uncertainties. Therefore, a minimum of 15% of the 95 IC 348 cluster members within the OVRO mosaic contain a detectable Ks-L excess indicative of an optically thick circumstellar disk.



Figure 5:
Left: Grayscale image of the lambda 3mm continuum emission toward IC 348. Darker regions represent bright intensities.

Right: Contour map of the OVRO mosaic. Contours begin at 3sigma above the RMS noise of 0.75 mJy/beam with increments of 1 sigma. The dotted boundary that encompasses the image shows the unit gain boundary of the mosaic (see Fig. 1). Open circles represent 95 probable members of the IC 348 cluster within the field of view of the OVRO mosaic that have been identified from spectroscopy (Herbig 1998; Luhman etal. 1998; Luhman 1999) and narrow band imaging (Najita etal. 2000). This figure shows that none of the known IC 348 stars were detected in the lambda 3mm continuum at the 3sigma noise level or greater.



Figure 6:
Frequency distribution of the observed flux densities at 98 GHz over the entire OVRO mosaic (solid circles) and toward the 95 IC 348 members (histogram). The frequency distribution indicated by the solid circles have been arbitrarily scaled by 1/600. The dotted curves through the circles and through the histogram show the expected distribution of fluxes in the two samples for gaussian noise with a dispersion of 0.75 mJy/beam, which is the average noise in the OVRO mosaic. This figure shows that the fluxes over the entire mosaic are consistent with gaussian noise. The mean flux observed toward the 95 IC 348 members is 0.22 ± 0.08 mJy, where the uncertainty is the standard deviation of the mean.


Figure 7:
Histogram of the effective dust temperatures of circumstellar disks in Taurus-Auriga. The effective dust temperature represents the dust temperature that, when combined with the observed lambda 1.3mm fluxes, reproduces the disk masses derived by fitting power-law distributions for the dust temperature and mass surface density to the observed spectral energy distribution of young stars in Taurus-Auriga (Beckwith etal. 1990; Osterloh & Beckwith 1995). Most stars have effective dust temperatures of about 20 K, which was used to convert the observed OVRO lambda 3mm fluxes to disk masses. Four stars have effective dust temperatures greater than 100 K and are not shown in this figure.