Spitzer observations of NGC 2547: The disk population at 25 million years

We present Spitzer observations of the young cluster NGC 2547, obtaining photometry at 3.6, 4.5, 5.8, 8.0, and 24 μm, reaching significantly fainter infrared sensitivities than previous studies. With these observations, we investigate the disk frequency in this cluster. A total of 3770, 2408, 1988, 1238, and 1123 sources were detected in the common region of the five respective bands. The detection limits were 14.9, 14.0, 13.5, 13.3, and 12.0 mag, respectively. The large majority of sources are associated with 2MASS objects. From this large ensemble, we utilize the criteria of Naylor et al. to identify 184 likely members of the cluster. The analyses in this paper are focused on these likely members. Of the 184 candidates, 162 were detected in at least one Infrared Array Camera (IRAC) band. These objects form a well-defined family in the J - K,K -[3.6] color-color diagram. There is no evidence for excess emission at 3.6 μm for cluster candidates with J - K < 0.8, corresponding to spectral types earlier than late K. For later type stars, only 12 have evidence for a 3.6 μm excess. Hence, we derive a 3.6 μm emitting disk fraction of less than 7%. The lack of excess for the more massive stars may indicate a difference in the disk dissipation timescales for different mass stars. At 24 μm, 32 of the sources are detected. Most of them have photospheric K -[3.6] and K -[24] colors, but approximately a quarter show an excess at 24 μm. This observation may be evidence for cool disks with central holes.