356 research outputs found
Revised analysis of SPIRE observations for 2M1207
We have revised our analysis of the SPIRE observations of 2MASSW
J1207334-393254 (2M1207). Recent PACS observations show a bright source located
~25" east of 2M1207. There are issues in terms of the detection/non-detection
of the bright source when comparing the Spitzer, WISE, and PACS observations.
It is apparently inconsistent, perhaps due to variability or low
signal-to-noise of the data. We have looked into the possible misidentification
of the target, and have revised the measured SPIRE fluxes and the disc
parameters for 2M1207. We have also reviewed which among the various formation
mechanisms of this system would still be valid.Comment: Revised SPIRE fluxe
Forever Young: High Chromospheric Activity in M subdwarfs
We present spectroscopic observations of two halo M subdwarfs which have H
alpha emission lines. We show that in both cases close companions are the most
likely cause of the chromospheric activity in these old, metal-poor stars. We
argue that Gl 781 A's unseen companion is most likely a cool helium white
dwarf. Gl 455 is a near-equal-mass M subdwarf (sdM) system. Gl 781 A is rapidly
rotating with v sin i = 30 km/s. The properties of the chromospheres and X-ray
coronae of these systems are compared to M dwarfs with emission (dMe). The
X-ray hardness ratios and optical chromospheric lines emission ratios are
consistent with those seen in dMe stars. Comparison to active near-solar
metallicity stars indicates that despite their low metallicity ([m/H] = -1/2),
the sdMe stars are roughly as active in both X-rays and chromospheric emission.
Measured by L_X/L_bol, the activity level of Gl 781 A is no more than a factor
of 2.5 subluminous with respect to near-solar metallicity stars.Comment: 16 pages including 1 figure, AASTeX, to appear in May 1998 A.
New Brown Dwarf Disks in the TW Hydrae Association
In our analysis of {\it Spitzer}/IRS archival data on the stellar and
sub-stellar members of the TW Hydrae Association (TWA), we have discovered two
new brown dwarf disks: a flat optically thick disk around SSSPM J1102-3431
(SSSPM 1102), and a transition disk around 2MASS J1139511-315921 (2M1139). The
disk structure for SSSPM 1102 is found to be very similar to the known brown
dwarf disk 2MASSW J1207334-393254 (2M1207), with excess emission observed at
wavelengths as short as 5 \micron. 2M1139 shows no excess emission shortward
of 20 \micron, but flares up at longer wavelengths, and is the first
transition disk detected among the sub-stellar members of TWA. We also report
on the {\it Spitzer}/70 \micron observations, and the presence of a weak {\it
absorption} 10 \micron silicate feature for 2M1207. The absorption can be
attributed to a close to edge-on disk at a 75\degr inclination. The 10
\micron spectrum for 2M1207 shows crystalline forsterite features, with a
peak in absorption near 11.3 \micron. No silicate absorption/emission is
observed towards SSSPM 1102. While only 6 out of 25 stellar members show excess
emission at these mid-infrared wavelengths, {\it all} of the TWA brown dwarfs
that have been observed so far with {\it Spitzer} show signs of disks around
them, resulting in a disk fraction of at least 60%. This is a considerable
fraction at a relatively older age of 10 Myr. A comparison with younger
clusters indicates that by the age of the TWA (10 Myr), the disk fraction
for brown dwarfs has not decreased, whereas it drops by a factor of 2 for
the higher mass stars. This suggests longer disk decay time scales for brown
dwarfs compared to higher mass stars.Comment: 23 pages, 6 figures. Accepted in Ap
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