Discovery of an old photoevaporating disk in σ Orionis

Astronomy & Astrophysics, Feb 2009

The photoevaporation of circumstellar disks is a powerful process in the disk dissipation at the origin of the Orion proplyds. This Letter reports the first detection of a photoevaporating disk in the final but long-lasting phase of its evolution. The disk is associated to a low-mass T Tauri member of the σ Ori Cluster. It is characterized by a very low (if any) accretion rate and by a tenuous (  /yr) photoevaporation wind, which is unambiguously detected in the optical spectrum of the object. The wind emits strong forbidden lines of [SII] and [NII] because the low-mass star is close to a powerful source of ionizing photons, the O9.5 star σ Ori.

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Discovery of an old photoevaporating disk in σ Orionis

A&A Discovery of an old photoevaporating disk in σ Orionis Lrte ot eht E. Rigliaco 1 2 A. Natta 2 S. Randich 2 G. Sacco 0 0 Osservatorio Astronomico di Palermo , INAF, Piazza del Parlamento 1, 90134 Palermo , Italy 1 Università di Firenze , Dipartimento di Astronomia, Largo E. Fermi 2, 50125 Firenze , Italy 2 Osservatorio Astrofisico di Arcetri, INAF , Largo E. Fermi 5, 50125 Firenze , Italy The photoevaporation of circumstellar disks is a powerful process in the disk dissipation at the origin of the Orion proplyds. This Letter reports the first detection of a photoevaporating disk in the final but long-lasting phase of its evolution. The disk is associated to a lowmass T Tauri member of the σ Ori Cluster. It is characterized by a very low (if any) accretion rate and by a tenuous (M˙ loss ∼ 10−9 M /yr) photoevaporation wind, which is unambiguously detected in the optical spectrum of the object. The wind emits strong forbidden lines of [SII] and [NII] because the low-mass star is close to a powerful source of ionizing photons, the O9.5 star σ Ori. stars; formation - accretion; accretion disks 1. Introduction The evolution of circumstellar disks surrounding low-mass T Tauri stars (TTS in the following) is controlled by the interplay of different physical processes, which include viscous accretion onto the central star, photoevaporation by the stellar radiation field, and planet formation. If the low-mass star is sufficiently close to a more massive and hotter object, photoevaporation by this external source of high-energy radiation is also important because it can dominate the disk evolution. The process of photoevaporation has been extensively discussed (see, e.g. Hollenbach et al. PPIV; Dullemond et al. PPV). High-energy photons heat gas disks to temperatures such that the thermal pressure exceeds the gravity from the central star. The disk evaporates from outside-in, with a mass-loss rate that decreases with time as the disk shrinks. Most of the well-studied Orion proplyds are caught in the first, short evolutionary phase, when the mass-loss rate is very high (M˙ loss > 10−7 M /yr, Henney et al. 1999). We report in this Letter on the first detection of a proplyd in a much later evolutionary phase, when the mass-loss rate is 10−8 M /yr. The object (a low-mass TTS) is located in the star-forming region σ Ori (age ∼ 2−3 Myr), which contains the massive quintuplet system σ Ori. The brightest star of this system (spectral type O9.5) forms a large, low-density HII region (Habart et al. 2005) and the bright PDR known as the Horsehead (Abergel et al. 2003) . The low background from the HII region and the relatively short distance (∼400 pc; Mayne & Naylor 2008) allow us to detect the slow photoevaporated TTS wind and to measure its optical line emission spectrum. 2. The T Tauri star SO587 2.1. Stellar properties SO587 (also identified as R053833-0236 Wolk 1996 or Mayrit 165257 Caballero 2008) (α2000 = +05:38:34.04, δ2000 = −02 : 36:37.33) has been classified as M3–M4 by Zapatero-Osorio et al. (2002) on the basis of low-resolution optical spectra (Teff ∼ 3300 K). The optical extinction is negligible (Oliveira et al. 2004) , and we estimate a luminosity of 0.3 ± 0.1 L , based on this spectral type and the V , R, I magnitudes (Wolk 1996) . The corresponding mass is about 0.2 M , both from D’Antona & Mazzitelli (1997) and Baraffe et al. (1998) evolutionary tracks. Based on its location on the HR diagram, SO587 has an age of ∼1 Myr, apparently slightly younger than the bulk of the σ Ori stars. SO587 is an X-ray source, with luminosity LX ∼ 1029 erg/s (Franciosini et al. 2006) 2.2. Disk properties S0587 is detected by Spitzer at all IRAC wavelengths and at 24 μm with MIPS. It shows a relatively weak excess emission, and is classified as an evolved disk (EV, Hernandez et al. 2007) . We confirm this classification. The IR emission is reproduced well by a geometrically flat, optically thick disk heated by the central star, seen at an inclination of about 40 deg. The outer disk radius is not constrained by the existing infrared photometry, which is limited to 24 μm, as long as Rout ∼> 1−2 AU. The model is not unique but exploration of a large number of disk models rules out optically thin disks, which have different Spitzer colors and lower luminosity than observed in SO587 (Ldisk/Lstar ∼ 0.07) (see Cieza et al. 2007) . Also, there is no evidence in the SED of the large inner holes (few AUs) seen in transitional disks (Chiang & Murray-Clay 2007, and references therein) , as all the models that fit the data have inner radii <∼0.1 AU. SO587 shows no evidence of significant accretion onto the central star. An estimate of M˙ acc can be obtained from the U band magnitude 18.5 ± 0.8 measured by Wolk (1996) . For the spectral type and luminosity of SO587, this corresponds to an excess U band emission of 1 mag at most. Using the correlation between the U band excess emission and the accretion luminosity (...truncated)


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E. Rigliaco, A. Natta, S. Randich, G. Sacco. Discovery of an old photoevaporating disk in σ Orionis, Astronomy & Astrophysics, 2009, pp. L13-L16, Volume 495, Issue 2, DOI: 10.1051/0004-6361/200811535