Weak-Line T Tauri Star Disks I. Initial Spitzer Results from the Cores to Disks Legacy Project
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Weak-line T Tauri Star Disks I. Initial Spitzer Results from the Cores to Disks Legacy Project Deborah L. Padgett1, Lucas Cieza2, Karl R. Stapelfeldt3, Neal J. Evans, II2, David Koerner4, Anneila Sargent5, Misato Fukagawa1, Ewine F. van Dishoek6, Jean-Charles Augereau6, Lori Allen7, Geoff Blake5, Tim Brooke5, Nicholas Chapman8, Paul Harvey2, Alicia Porras7, Shih-Ping Lai8, Lee Mundy8, Philip C. Myers7, William Spiesman2, Zahed Wahhaj4 ABSTRACT Using the Spitzer Space Telescope, we have observed 90 weak-line and clas- sical T Tauri stars in the vicinity of the Ophiuchus, Lupus, Chamaeleon, and Taurus star-forming regions as part of the Cores to Disks (c2d) Spitzer Legacy project. In addition to the Spitzer data, we have obtained contemporaneous op- tical photometry to assist in constructing spectral energy distributions. These objects were specifically chosen as solar-type young stars with low levels of Hα emission, strong X-ray emission, and lithium absorption i.e. weak-line T Tauri stars, most of which were undetected in the mid-to-far IR by the IRAS survey. Weak-line T Tauri stars are potentially extremely important objects in determin- ing the timescale over which disk evolution may take place. Our objective is to determine whether these young stars are diskless or have remnant disks which arXiv:astro-ph/0603370v1 14 Mar 2006 are below the detection threshold of previous infrared missions. We find that only 5/83 weak-line T Tauri stars have detectable excess emission between 3.6 and 70 µm which would indicate the presence of dust from the inner few tenths of an AU out to the planet-forming regions a few tens of AU from the star. Of these sources, two have small excesses at 24 microns consistent with optically thin disks; the others have optically thick disks already detected by previous IR surveys. All of the seven classical T Tauri stars show excess emission at 24 and 70 µm although their properties vary at the shorter wavelengths. Our initial results show that disks are rare among young stars selected for their weak Hα emission. Subject headings: stars: pre-main sequence — (stars:) planetary systems: protoplane- tary disks — infrared: stars 1 1. Introduction same spectral type (Covino et al. 1997). The presence of a strong lithium absorp- T Tauri stars (Joy 1945) have long been tion indicates stellar youth among stars known as solar-mass pre-main sequence with deeply convecting photospheres since stars. Sprinkled throughout molecular lithium is easily destroyed at high temper- cloud complexes, or gathered in dense clus- atures. In practice, using the Li I EW to ters, they have traditionally been identified select young stars is a difficult task, since by their unusual optical spectra which in- the strength of the lithium line may de- clude bright emission lines of Hα and other pend on the rotational history of the star permitted and forbidden atomic transi- and spot coverage (Mendes et al. 1999). In tions. However, with the advent of X-ray addition, spectral classes earlier than G5 molecular cloud surveys, another class of may retain strong lithium lines through- solar-type young star has been identified out their main sequence lifetimes (Spite et from their high X-ray luminosity. Unlike al. 1996). Thus, current lists of WTTS the spectroscopically identified “Classical” are probably contaminated to some degree T Tauri stars (CTTS), these objects lack by young main sequence stars which are ˚ Hα emission in excess of 10 A in equiva- closer than the cloud, but too faint for ac- lent width (EW) and are thus known as curate parallax measurements (Alcala et “weak-line” T Tauri stars (WTTS; Her- al. 1998). big & Bell 1988). Confirmation that X- The current paradigm of planet forma- ray identified WTTS are young stars re- tion is still based on the venerable IRAS quires high resolution optical spectra that (Strom et al. 1989) and millimeter contin- show a solar-type spectrum with a Li I uum surveys (Beckwith et al. 1990) of the 6707 A˚ absorption line equivalent width late 1980’s that found mid-to-far-infared stronger than that of a Pleiades star of the and/or millimeter excess emission around 1Spitzer Science Center, MC220-6, Califor- about half of solar-type stars in nearby nia Institute of Technology, Pasadena, CA 91125 clouds. This picture of disk evolution [email protected] starts with Class 0-I sources which are ex- 2Astronomy Departement, University of Texas, tremely faint shortward of 25 microns, pro- 1 University Station C1400, Austin, TX 78712 ceeds through Class I with a rising IR Spec- 3 Jet Propulsion Laboratory, MS 183-900, 4800 tral Energy Distribution (SED), continues Oak Grove Drive, Pasadena, CA 91109 to Class II which has a flat or falling SED 4Northern Arizona University, Department of Physics and Astronomy, Box 6010, Flagstaff, AZ with strong IR excesses above the photo- 86011 sphere, and ends at Class III where the 5California Institute of Technology, Pasadena, excess emission indicates a tenuous, opti- CA 91125 cally thin disk (Adams, Lada, & Shu 1987; 7Smithsonian Astrophysical Observatory, 60 Andr´e& Montmerle 1994). In practice, Garden Street, MS42, Cambridge, MA 02138 IRAS and ISO were not sensitive enough 8 Astronomy Departement, University of Mary- to detect optically thin disks in the mid- land, College Park, MD 20742 IR at the distance of nearby star formation 6Leiden Observatory, Postbus 9513, 2300 R.A. Leiden, Netherlands regions; however, the Spitzer Space Tele- 2 scope does have this capability. Almost not retain remnant circumstellar mate- as soon as the first WTTS were identi- rial and did not possess substantial disks fied, they were presumed to be the Class long enough for plausible planet formation. III “missing link” of the planet formation Typical “core accretion” models of giant paradigm since “post” T Tauri stars had planet formation require a minimum of 5 proved elusive. Initial ground-based NIR million years to make a Jupiter, most of and MIR studies showed that inner disks which is spent in the coagulation of dust were very rare among the WTTS popula- and accretion of planetesimals (Wieden- tion (Skrutskie et al. 1990). More recent schilling 2000), although accretion times JHKL studies of several young clusters in- continue to drop in the most recent models dicated that inner disks disappear in 5 - 6 (Goldreich et al. 2004a,b) and the alterna- Myr (Haisch, Lada, & Lada 2001). On the tive “disk instability” model can produce other hand, a JHKL study of the ≈ 10 Myr giant planets on a very short timescale η Cha cluster found a rather high disk frac- (Boss 2001). A low detection frequency of tion among its few known members (Lyo any infrared excess among WTTS would et al. 2003), although a smaller percent- provide evidence for diskless young stars. age of disks was found by Haisch et al. In addition, this study will address the (2005). Both IRAS and millimeter surveys dispersion in disk properties among coeval indicate that optically thick outer disks are stars in the same environment. These dif- rare among WTTS (Strom et al. 1989, ferences could possibly be related to the Osterloh & Beckwith 1995). However, a early formation of planets in the stars sensitive study of tenuous material in the where the disks have inner holes or have terrestrial planet forming zone of WTTS disappeared entirely. Such a hypothesis is has awaited the power of the Spitzer Space potentially testable with future high res- Telescope (Spitzer). Spitzer has over 1000 olution astrometric studies of young stars times the sensitivity at 24 µm of the IRAS with and without disks to see if the pres- satellite, with similar improvements in its ence of giant planets is related to the state other imaging bands (Werner et al. 2004). of disk evolution. Only Spitzer can distinguish between tran- Several Spitzer surveys to assess the de- sitional disks (optically thick with inner tection frequency of excess in young stars holes), optically thin remnant disks with are either in progress or have been pub- low disk luminosity Ld (Ld/L∗ << 1, like lished recently. The entire range of Spitzer the debris disks found around older stars), instrumentation are utilized by these stud- and completely diskless “naked” T Tauri ies, including the 3.6 - 8.0 µm camera stars. (IRAC), the 24, 70, and 160 µm photome- Study of circumstellar material around ter (MIPS), and the 5 - 40 µm spectro- WTTS promises to answer some significant graph (IRS). There are several investiga- questions about planet formation around tions of excess among very young stars in low mass stars. The first is whether some individual star-forming regions (Reach et young stars are born “diskless”; that is, al. 2004, Muzerolle et al. 2004, Padgett some stars at the age of 1 - 3 Myr do et al. 2004, Young et al. 2005, Hart- 3 mann et al. 2005a, etc.). These stud- 70 µm sensitivities are good. Although ies investigate known young star popula- their youngest age group includes solar- tions including WTTS, CTTS, and Class type stars of similar age to the current I sources. Among older pre-main sequence WTTS sample, the FEPS young stars are associations, Chen et al. (2005) report on located more than 6◦ from nearby clouds a MIPS 24 & 70 µm survey of 40 F- and (Silverstone et al. 2006). In contrast to G- type stars in the Sco-Cen OB associ- these surveys, our 0.5 - 70 µm survey fo- ation with an age of 5 - 20 Myr (Chen cuses only on WTTS in and adjacent to et al. 2005). The Megeath et al. (2005) several star-forming clouds within 200 pc.