Timescale of mass accretion in pre-main-sequence stars

We present the initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0-M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of the ages of between 1-30 Myr. The fraction of accreting stars decreases from ~60% at 1.5-2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of 10 -11 M ⊙ yr -1 We compared the fraction of stars showing ongoing accretion (f acc ) to the fraction of stars with near-to-mid infrared excess (f IRAC ). In most cases we find f acc < F IRAC , i.e., mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of ≳10 -11 M ⊙ yr -1 while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (τ acc ) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (τ IRAC ) of 3 Myr. Planet formation and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.

[1]  T. Henning.  Early phases of planet formation in protoplanetary disks , 2008 .

[2]  Ben Zuckerman,et al.  Young Stars Near the Sun , 2004 .

[3]  R. A. Gutermuth,et al.  The Disk Population of the Chamaeleon I Star-forming Region , 2008, 0803.1019.

[4]  Douglas Forbes.  Star Formation in NGC 6531-Evidence From the age Spread and Initial Mass Function , 1996 .

[5]  K. Covey,et al.  A SPECTROSCOPIC STUDY OF YOUNG STELLAR OBJECTS IN THE SERPENS CLOUD CORE AND NGC 1333 , 2009, 0904.1244.

[6]  Jack J. Lissauer,et al.  Formation of the Giant Planets by Concurrent Accretion of Solids and Gas , 1995 .

[7]  S. E. Dahm,et al.  Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr , 2007 .

[8]  Alan P. Boss,et al.  Giant Planet Formation by Gravitational Instability , 1997 .

[9]  Lynne A. Hillenbrand,et al.  UV Excess Measures of Accretion onto Young Very Low Mass Stars and Brown Dwarfs , 2008, 0801.3525.

[10]  N. V. Kharchenko,et al.  Astrophysical parameters of Galactic open clusters , 2005 .

[11]  C. Clarke,et al.  The dispersal of circumstellar discs: the role of the ultraviolet switch , 2001 .

[12]  F. Rasio,et al.  Gas Disks to Gas Giants: Simulating the Birth of Planetary Systems , 2008, Science.

[13]  D. Nguyen,et al.  A Comprehensive View of Circumstellar Disks in Chamaeleon I: Infrared Excess, Accretion Signatures, and Binarity , 2007, 0708.0266.

[14]  J. Alves,et al.  Constraints on inner disk evolution timescales : A disk census of the η chamaeleontis young cluster , 2004, astro-ph/0506352.

[15]  J. Caballero.  Spatial distribution of stars and brown dwarfs in σ Orionis , 2007, 0710.1255.

[16]  Subhanjoy Mohanty,et al.  The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs , 2005, astro-ph/0502155.

[17]  G. Rieke,et al.  Spitzer Observations of IC 348: The Disk Population at 2-3 Million Years , 2005, astro-ph/0511638.

[18]  Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr Old Upper Scorpius OB Association , 2006, astro-ph/0609372.

[19]  E. Young,et al.  A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster , 2007, astro-ph/0701476.

[20]  Elizabeth A. Lada,et al.  Disk Frequencies and Lifetimes in Young Clusters , 2001, astro-ph/0104347.

[21]  L. Allen,et al.  Moderate-resolution spectral standards from lambda 5600 to lambda 9000 , 1995 .

[22]  Alan P. Boss,et al.  Evolution of the Solar Nebula. IV. Giant Gaseous Protoplanet Formation , 1998 .

[23]  Lithium and halpha in stars and brown dwarfs of sigma orionis , 2002, astro-ph/0202147.

[24]  High-Resolution Spectroscopy in Tr 37: Gas Accretion Evolution in Evolved Dusty Disks* , 2006, astro-ph/0607534.

[25]  S. Edwards,et al.  Helium Emission from Classical T Tauri Stars: Dual Origin in Magnetospheric Infall and Hot Wind , 2001 .

[26]  G. Fazio,et al.  IRAC Observations of Taurus Pre-Main-Sequence Stars , 2005, astro-ph/0505323.

[27]  L. Hartmann,et al.  Accretion and the Evolution of T Tauri Disks , 1998 .

[28]  Caltech,et al.  Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga , 2002, astro-ph/0209164.

[29]  Disk-dispersal and planet-formation timescales , 2008, 0805.0386.

[30]  L. Hartmann,et al.  A Search for Very Low Mass Pre-Main-Sequence Stars in Taurus , 1998 .

[31]  Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation , 2005, astro-ph/0506251.

[32]  E. Feigelson,et al.  THE LONG-LIVED DISKS IN THE η CHAMAELEONTIS CLUSTER , 2009, 0906.3365.

[33]  L. Hartmann,et al.  Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr , 2005 .

[34]  The Keele-Exeter young cluster survey - I. Low-mass pre-main-sequence stars in NGC 2169 , 2006, astro-ph/0611630.

[35]  K. Horne,et al.  AN OPTIMAL EXTRACTION ALGORITHM FOR CCD SPECTROSCOPY. , 1986 .

[36]  K. Flaherty,et al.  EVIDENCE FOR EARLY CIRCUMSTELLAR DISK EVOLUTION IN NGC 2068/71 , 2007, 0712.1601.

[37]  Y. Alibert,et al.  Migration and giant planet formation , 2004, astro-ph/0403574.

[38]  The lithium depletion boundary and the age of NGC 2547 , 2003, astro-ph/0303083.

[39]  Usa,et al.  An Empirical Criterion to Classify T Tauri Stars and Substellar Analogs Using Low-Resolution Optical Spectroscopy , 2003, astro-ph/0309284.

[40]  L. Testi,et al.  Accretion in brown dwarfs: An infrared view , 2004 .

[41]  Binarity as a key factor in protoplanetary disk evolution : Spitzer disk census of the η chamaeleontis cluster , 2006, astro-ph/0610853.

[42]  U. Michigan,et al.  25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a , 2007, astro-ph/0701710.

[43]  J. Loon,et al.  Circumstellar discs in the young σ Orionis cluster , 2006, astro-ph/0603262.

[44]  U. Toronto,et al.  Lithium Depletion of Nearby Young Stellar Associations , 2008, 0808.3584.

[45]  Alexis Brandeker,et al.  Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability , 2006, astro-ph/0605601.

[46]  Willy Benz,et al.  Models of giant planet formation with migration and disc evolution , 2004 .