José Antonio Caballero1
Eduardo Lorenzo Martín1
María Rosa Zapatero Osorio2
Víctor Javier S. Béjar1
Rafael Rebolo1,3
.
gotzon
18-1-95
.
1: Instituto de Astrofísica de Canarias
2: LAEFF-INTA
3: Consejo Superior de Investigaciones Científicas
Introduction: T Tauri stars and brown dwarfs. T Tauri stars are the bridge between recently born embedded protostars and zero
age main sequence stars. They are very young objects, with ages lower than 20-30 Myr. They are found in young open clusters
or in nearby star forming regions. According to the strength of the Balmer lines, they are classified as Classical T Tauri Stars (CTTS)
or Weak-line T Tauri Stars. The most prominent characteristics of the CTTS are the presence of lithium in absorption and the
existence of strong emission lines in their spectra, especially the Ha line and the Ca II H&K doublet. Several CTTS also display
photometric variability, ultraviolet and/or infrared excess, optical veiling, strong X-ray emission, winds or jets. The best scenario
to explain all these features is the magnetospheric accretion of mass infalling from a surrounding circumstellar disc. Bertout
(1988), Appenzeller & Mundt (1989) or Koenigl (1991) have reviewed the properties of T Tauri stars.
The T Tauri scenario has been considered, in general, for stars in the mass range between 3 and 0.2 Msol. However, the existence of a T
Tauri phase in objects below the hydrogen burning limit (~0.072 Msol for solar metallicity) suggested by several authors during the
past few years (see, for example, Barrado y Navascués & Martín 2003 and references therein) has raised a lot of interest.
S Ori J053825.4-024241:
A Classical T Tauri-like
object in the
substellar limit
S Ori J053825.4-024241. It was first detected by Béjar
et al. (2004) in an RI survey of the young (2-4 Myr)
nearby (~350 pc) σ Orionis cluster. Soon after,
Caballero et al. (2004) found S Ori J053825.4-024241
to be a highly photometric variable object. It
displayed photometric variations of ~0.36 mag in less
than two days, and of ~0.09 mag in less than two
hours, having the largest amplitude of variation in
the sample of 28 brown dwarfs examined. A strong
near infrared excess in the Ks band was also
detected.
Abstract: We present photometric and
spectroscopic results on S Ori J053825.4024241, an M 6.0±0.5 dwarf in the frontier
between stars and brown dwarfs of the
young (2-4 Myr) nearby (~350 pc) σ Orionis
open cluster.
Observations. Photometric monitoring in the I band
and in white light using 1 m-class telescopes and
spectroscopic observations of different resolutions
were performed during the 2003-04 winter. We have
used data collected at the
! 0.82-m IAC-80,
! the 1.0-m Orbital Ground Station,
! the 1.55-m Telescopio Carlos Sánchez,
! the 2.5-m Isaac Newton Telescope,
! the 2.56-m Nordic Optical Telescope,
! the 3.5-m Calar Alto Teleskop,
! the 8-m Very Large Telescope Ut1 Antu and
! the 10-m Keck I Telescope .
Caballero et al. (2004) showed that it is
strongly photometrically variable and has
near infrared excess in the Ks band.
Here, we confirm its optical variability, with
an amplitude of about half a magnitude,
which is extraordinary large for such a low
mass object (between 0.06 and 0.08
Msolar). Variations in timescales of hours
and days have been detected.
Additionally, it displays strong Hα emission,
with a persistent pseudo-equivalent width
of more than 200 Å, possibly associated to
mass accretion from a surrounding disc.
Other emission lines, including forbidden
lines, which are usually associated to
outflows, have been detected in a medium
resolution spectrum.
S Ori J053825.4-024241 appears to be one of
the least massive Classical T Tauri-like
objects ever detected.
Optical photometric variability (four figures, below)
Top left: I-band lightcurve of S Ori J053825.4-024241 (bottom, red stars) and of a
reference star in the same field of view (top, small blue dots). Data were taken
with the CCD at the IAC-80. Vertical scale is similar in both plots. The lightcurve of
S Ori J053825.4-024241 shows a big magnitude decrease that lasted for one
week.
Top right: Same as previous figure, but zooming the two nights in which the object
was monitored during about 2.5 h. An additional lower-amplitude short-term
variability pattern can be evaluated, superimposed to the main cycle.
Bottom left: Lightcurve in white light of S Ori J053825.4-024241 (red stars) and of a
reference star in the same field of view (small blue dots). Data were taken with
ESACCD at the OGS. It is seen the big dip around 30 January 2003, as shown in the
IAC-80 plots.
Bottom right: Same as previous figure, but zooming the four consecutive nights.
The superimposed lower-amplitude variability pattern with timescales of hours is
clearly seen.
Medium and low resolution spectroscopy (two figures below)
Top: Low resolution spectrum of S Ori J053825.4-024241 taken with ALFOSC at
the NOT Telescope on 26 Dec 2003. Spectral coverage is 5500 to 9000 Å.
From this spectrum we infer a spectral type of M 6.0±0.5. Hα is in emission with
a pseudo-equivalent width of ~210 Å and FWHM > 200 km/s. Such a velocity
is higher than expected for a rotational period (otherwise the object would
be disrupted). We interpret the large FWHM to be
associated to a
surrounding accretion disc or ejection of material.
Bottom: Part of the wavelength-calibrated mid-resolution spectrum taken
with LRIS at the Keck I Telescope on 23 Jan 2004, centered in 6300 Å. Besides
Hα (pEW(Hα) ~285 Å), we have detected the He I D3 λ5876Å triplet and the
forbidden lines [O I] λ6300 Å and possibly [N II] λ6583 Å. We can establish only
an upper limit for the pseudo-equivalent width of Li I λ6708 Å (<0.2 Å). This line
may be severely affected by disk veiling. Our Vrad measurement is consistent
with the σ Orionis cluster systemic velocity. The [O I] emission line (which is not
a photospheric line) is blue-shifted with respect the mean velocity of the
object. This effect is also observed in CTTSs with jets pointing toward us,
supporting the disc scenario.
Right: J-band image
of the field around S
Ori J053825.4-024241
taken with the ISAAC
instrument at the Very
Large Telescope UT1
Antu on 10 Dec 2001.
Size is roughly 2.5
arcmin x 4.9 arcmin. S
Ori J053825.4-024241 is
marked with a yellow
square. The bright HD
294273 star, 1 arcmin
to the South East of S
Ori J053825.4-024241,
is an A 3 star that
presumably belong to
the σ Orionis cluster
(based on colour and
spectroscopic data).
Projected physical
separation is ~20 000
AU at the distance of
the cluster.
!
Conclusions. We confirm the Classical T Tauri-like nature of S Ori J053825.4024241 through an intensive optical photometric monitoring and low and
intermediate resolution spectra.
Several observational data suggest that this young very low mass M 6 dwarf is
surrounded by an accretion disc. We have detected long-term photometric
variations of almost 0.5 mag in time scales of one week and short-term
variability of ~0.1 in scale of hours. Two measurements of the Hα pseudoequivalent width at two different epochs give values larger than 200 Å. We
have also detected several emission lines, including forbidden lines of
oxygen, that are characteristic of T Tauri stars.
New photometric and spectroscopic observations are ongoing.
Quantity
R.A.
Dec.
R
I
J
H
Ks
Sp. Type
pEW(Hα)
pEW(Li I)
pEW([O I])
H-Ks vs. J-H colour-colour digram of
objects of the σ Orionis cluster. S Ori
J053825.4-024241 displays a strong
nIR excess (from Caballero et al.
Value
05 38 25.44
-02 42 41.2
18.66
16.86
14.88
14.16
13.57
M 6.0±0.5
210-285 Å
<0.2 Å
2.3±0.5 Å
2.2±0.5 Å
!
!
!
!
Appenzeller Mundt, R.
1989, A&ARv, 1, 291
Barrado y Navascués,
D., Martín, E. L. 2003, AJ,
126, 2997
Béjar, V. J. S., Zapatero
Osorio, M. R., Rebolo, R.
2004, Astron. Nachr., in
press
Caballero, J. A., Béjar,
V. J. S., Rebolo, R,
Zapatero Osorio, M. R.,
2004, A&A, in press
Koenigl, A. 1991, ApJ,
370, L39
Table. We give the J2000.0 coordinates, average RIJHKs
magnitudes and the measurements of the pseudo-equivalent
widths of several lines. The object is variable in most of the bands.