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)