What Spitzer Missed: New Members of Lupus 3 that Don't Boast Their Youth


First Author:
Fernando Comeron
Email: fcomeron AT eso.org
ESO
KArl-Schwarzschild-Str. 2
D-85748 Garching bei Muenchen, Germany
Coauthors:
Spezzi, Loredana, ESA

Abstract

Spitzer surveys of star forming regions have produced the most complete census of their young stellar and substellar content available to date, thanks to their sensitivity to the emission of circumstellar dust heated to a broad range of temperatures. The majority of members already known thanks to other signatures of youth before the Spitzer era have been recovered by Spitzer based on color-color criteria, which in turn have revealed large numbers of previously unknown members. It is worth wondering therefore if there may be any remaining members unknown, undetected by any of the techniques based on the various signatures of youth usually displayed by members of star forming regions. We present here the results of a survey of the Lupus 1, 3, and 4 clouds that was conceived to provide ground-based visible/red imaging in support to the Spitzer ``Cores to Disks'' Legacy Program and that, combined with 2MASS photometry, allows us to identify late-type members of those star forming regions based solely on their photospheric spectral energy distributions. Our survey is thus especially well suited to the detection of low-mass stars and brown dwarfs whose circumstellar emission is either too faint to be detected by Spitzer, or altogether absent. The results reveal a significant number of objects with estimated temperatures generally below 3000 K, and visible to near-infrared spectral energy distributions well consistent with membership in Lupus. We present statistical arguments that demonstrate that confusion with either foreground or background contamination is effectively ruled out. The existence of this elusive population suggests that a non-negligible fraction of low mass stars and brown dwarfs manage to reduce their signatures of youth associated with accretion or with the presence of dusty disks to undetectable levels within the first few million years of their lives.
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