Spitzer c2d results on disk types, time-scales and disk evolution from 1 to 10 Myrs

Bruno Merín1, Luke Maud1, Hervé Bouy1, Loredana Spezzi2, Isa Oliveira3, Joanna M. Brown4, Ewine F. van Dishoeck3,4, Neal J. Evans II5, Lucas Cieza6, Karl Stapelfeldt8, Juan M. Alcalá9,

Paul M. Harvey5, David W. Koerner10, Lori E. Allen11, Luisa M. Rebull12

Introduction

The Spitzer data, due to its wavelength coverage and sensitivity, o ers an exceptional tool for studying the status and evolution of ffthe inner several AUs of the disks around young stars, were planets could eventually form. The ‘Cores to Disks’ (c2d) Spitzer Legacy Program (Evans et al. 2009) provided a large and homogeneous magnitude- limited sample of 1024 Young Stellar Objects. The estimated ages of these objects range between 1 to 10 Myrs, which encompasses nicely the observed time-scale for disk dissipation around low-mass stars. In this contribution we will classify this sample in disk types (primordial, settled and transitional) and will study their dependence with the stellar mass and age. Their numbers and time-scales will also be studied statistically with the aim of finding constraints to the di erent disk ffdispersal mechanisms, including the formation of planetary systems.

The data: The Cores to Disks (c2d) Spitzer Legacy program

The ‘Cores to Disks’ (c2d) Spitzer Legacy Program (Evans et al. 2009) observed five large, nearby young molecular clouds with IRAC and MIPS and produced a large and homogeneous magnitude- limited sample of 1024 Young Stellar Objects, out of which 773 are Class II or III objects (Evans et al. 2009).

24.0, 8.0, 4.5 μm

Acknowledgements

This work was partially made possible thanks to the ESA Trainee

and Research Fellowship programs at ESAC (Spain).

The method: disk classification and stellar parameters We define three main types of Spitzer disks by comparison with the median SED of the T Tauri stars in Taurus and a photosphere.

Discussion and Conclusions

• Although objects in the c2d YSO sample have a standard range of stellar masses for low-mass star forming regions, the stellar ages are found to be larger and more spread than those found in star-forming clusters.

• Comparison of the frequencies of disk types by mass and age of the c2d sample reveals:o No significant differences in disk types at low (< 0.5 Msun) or intermediate masses (0.5 - 1.5 Msun) for objects as old as 10 – 20 Myr.

o A significant difference in disk types for massive (> 1.5 Msun) stars, for which disks seem to evolve in much faster time-scales

• Since only marginal differences are found for the higher mass bin, we speculate that either the formation mechanism or the mantained presence of magnetic fields yield longer time-scales for disks around low-mass stars.

Figure 1 Left: Color composite images of some of the c2d-observed star-forming clouds using IRAC 2 (4.5 μm) in blue, IRAC 4 (8.8 μm) in green and MIPS 1 (24 μm) in red (except NGC 1333, with IRAC1, IRAC2 and IRAC4). The coverage of the c2d mosaics greatly extends the area of the well-known star-forming cluster above. These observations produced a magnitude-limited sample of 1024 so-called c2d YSOs (Evans et al. 2009).

Figure 2: Spectral Energy Distribution of the three main types of disks considered for this work. The open and filled circles are observed and dereddened photometry, respectively. The dotted line is the stellar photosphere and the dashed line is the median SED of the Classical T Tauri stars in Taurus (CTTS). Primordial and Settled disks are defined as objects with the majority of its photometry within or below 40% of the mean CTTS SED, respectively. Transitional disks are settled disks with larger MIPS1 or MIPS2 fluxes than the IRAC4 flux.

Primordial

Star

Disk TransitionalMedian CTT SED

Settled

Results 1: Stellar properties of the Spitzer YSOs We calculate the stellar luminosities integrating the stellar photospheres and effective temperatures from the spectral types. We then get masses and ages using the Baraffe et al. (1998) and Siess et al. (2004) PMS tracks.

Results 2: Disk properties as a function of stellar properties We show the sample in bins of stellar mass with different stellar structure (fully convective: M* < 0.5 Msun, radiative core + convective envelope: M*= 0.5 – 1.2 Msun, and convective envelope + radiative core: M* > 1.5 Msun) and age. Disk types in are shown as: Primordial, Transitional and Settled.

IC 348 Perseus Rho Ophiuchi

Serpens CoreNGC 1333 Perseus

Age 0-5 Myr Age 5-10 Myr Age 10-20 Myr Figure 3: Stellar mass and age distributions for the c2d YSO sample (class Flat, II and III only) for which a result for age/mass could be found. This sample contains 358 objects , which is a representative sample of all objects in these five clouds for which optical photometry and/or spectral types were available. (figure to be done nicer)

Figure 4: Left column Schematic view of the different stellar interiors for stars with different stellar mass ranges. Second to fourth columns Pie charts showing the frequency of the three main types of disks (Primordial in pale grey, transitional in grey and settled in dark grey) as a function of the stellar mass and age. Fisher’s statistical tests on the significance of the differences reveals that the distributions are indistinguishable at earlyand intermadiate ages (0-5 and 5-10 Myrs) but that low and very-low mass stars differ from massive stars in the late ages (10-20 Myrs) at the 71% confidence leve.