144 research outputs found
Binary Formation Mechanisms: Constraints from the Companion Mass Ratio Distribution
We present a statistical comparison of the mass ratio distribution of
companions, as observed in different multiplicity surveys, to the most recent
estimate of the single object mass function (Bochanski et al. 2010). The main
goal of our analysis is to test whether or not the observed companion mass
ratio distribution (CMRD) as a function of primary star mass and star formation
environment is consistent with having been drawn from the field star IMF. We
consider samples of companions for M dwarfs, solar type and intermediate mass
stars, both in the field as well as clusters or associations, and compare them
with populations of binaries generated by random pairing from the assumed IMF
for a fixed primary mass. With regard to the field we can reject the hypothesis
that the CMRD was drawn from the IMF for different primary mass ranges: the
observed CMRDs show a larger number of equal-mass systems than predicted by the
IMF. This is in agreement with fragmentation theories of binary formation. For
the open clusters {\alpha} Persei and the Pleiades we also reject the IMF
random- pairing hypothesis. Concerning young star-forming regions, currently we
can rule out a connection between the CMRD and the field IMF in Taurus but not
in Chamaeleon I. Larger and different samples are needed to better constrain
the result as a function of the environment. We also consider other companion
mass functions (CMF) and we compare them with observations. Moreover the CMRD
both in the field and clusters or associations appears to be independent of
separation in the range covered by the observations. Combining therefore the
CMRDs of M and G primaries in the field and intermediate mass primary binaries
in Sco OB2 for mass ratios, q = M2/M1, from 0.2 to 1, we find that the best
chi-square fit follows a power law dN/dq \propto q^{\beta}, with {\beta} =
-0.50 \pm 0.29, consistent with previous results.Comment: 9 pages, 9 figures, Accepted for publication in The Astrophysical
Journa
Mapping the Shores of the Brown Dwarf Desert II: Multiple Star Formation in Taurus-Auriga
We have conducted a high-resolution imaging study of the Taurus-Auriga
star-forming region in order to characterize the primordial outcome of multiple
star formation and the extent of the brown dwarf desert. Our survey identified
16 new binary companions to primary stars with masses of 0.25-2.5 Msun, raising
the total number of binary pairs (including components of high-order multiples)
with separations of 3--5000 AU to 90. We find that ~2/3--3/4 of all Taurus
members are multiple systems of two or more stars, while the other ~1/4--1/3
appear to have formed as single stars; the distribution of high-order
multiplicity suggests that fragmentation into a wide binary has no impact on
the subsequent probability that either component will fragment again. The
separation distribution for solar-type stars (0.7--2.5 Msun) is nearly log-flat
over separations of 3--5000 AU, but lower-mass stars (0.25--0.7 Msun) show a
paucity of binary companions with separations of >200 AU. Across this full mass
range, companion masses are well described with a linear-flat function; all
system mass ratios (q=M_B/M_A) are equally probable, apparently including
substellar companions. Our results are broadly consistent with the two expected
modes of binary formation (freefall fragmentation on large scales and disk
fragmentation on small scales), but the distributions provide some clues as to
the epochs at which the companions are likely to form.Comment: Accepted to ApJ; 32 pages, 7 figures, 6 tables in emulateapj forma
A Near-Infrared Spectroscopic Survey of Class I Protostars
We present the results of a near-IR spectroscopic survey of 110 Class I
protostars observed from 0.80 microns to 2.43 microns at a spectroscopic
resolution of R=1200. We find that Class I objects exhibit a wide range of
lines and the continuum spectroscopic features. 85% of Class I protostars
exhibit features indicative of mass accretion, and we found that the veiling
excess, CO emission, and Br Gamma emission are closely related. We modeled the
spectra to estimate the veiling excess (r_k) and extinction to each target. We
also used near-IR colors and emission line ratios, when available, to also
estimate extinction. In the course of this survey, we observed the spectra of
10 FU Orionis-like objects, including 2 new ones, as well as 3 Herbig Ae type
stars among our Class I YSOs. We used photospheric absorption lines, when
available, to estimate the spectral type of each target. Although most targets
are late type stars, there are several A and F-type stars in our sample.
Notably, we found no A or F class stars in the Taurus-Auriga or Perseus star
forming regions. There are several cases where the observed CO and/or water
absorption bands are deeper than expected from the photospheric spectral type.
We find a correlation between the appearance of the reflection nebula, which
traces the distribution of material on very large scales, and the near-IR
spectrum, which probes smaller scales. The spectra of the components of
spatially resolved protostellar binaries tend to be very similar. In particular
both components tend to have similar veiling and H_2 emission, inconsistent
with random selection from the sample as a whole. There is a strong correlation
between [Fe II] and H_2 emission, supporting previous results showing that H_2
emission in the spectra of young stars is usually shock excited by stellar
winds.Comment: 89 pages, 13 figures, 7 Table
The Initial Mass Function and Disk Frequency of the Rho Ophiuchi Cloud: An Extinction-Limited Sample
We have completed an optical spectroscopic survey of an unbiased,
extinction-limited sample of candidate young stars covering 1.3 square degrees
of the Rho Ophiuchi star forming region. While infrared, X-ray, and optical
surveys of the cloud have identified many young stellar objects (YSOs), these
surveys are biased towards particular stages of stellar evolution and are not
optimal for studies of the disk frequency and initial mass function.We have
obtained over 300 optical spectra to help identify 135 association members
based on the presence of H-alpha in emission, lithium absorption, X-ray
emission, a mid-infrared excess, a common proper motion, reflection nebulosity,
and/or extinction considerations. Spectral types along with R and I band
photometry were used to derive effective temperatures and bolometric
luminosities for association members to compare with theoretical tracks and
isochrones for pre-main-sequence stars. An average age of 3.1 Myr is derived
for this population which is intermediate between that of objects embedded in
the cloud core of Rho Ophiuchi and low mass stars in the Upper Scorpius
subgroup. Consistent with this age we find a circumstellar disk frequency of
27% plus or minus 5%. We also constructed an initial mass function for an
extinction-limited sample of 123 YSOs (A_v less than or equal to 8 mag), which
is consistent with the field star initial mass function for YSOs with masses >
0.2 M_sun. There may be a deficit of brown dwarfs but this result relies on
completeness corrections and requires confirmation.Comment: 46 pages, 7 figures, 4 table
A Keck LGS AO Search for Brown Dwarf and Planetary Mass Companions to Upper Scorpius Brown Dwarfs
We searched for binary companions to 20 young brown dwarfs in the Upper
Scorpius association (145 pc, 5 Myr, nearest OB association) with the the Laser
Guide Star adaptive optics system and the facility infrared camera NIRC2 on the
10 m Keck II telescope. We discovered a 0.14" companion (20.9+-0.4 AU) to the
<0.1 MSun object SCH J16091837-20073523. From spectral deconvolution of
integrated-light near-IR spectroscopy of SCH1609 using the SpeX spectrograph
(Rayner et al. 2003), we estimate primary and secondary spectral types of
M6+-0.5 and M7+-1.0, corresponding to masses of 79+-17 MJup and 55+-25 MJup at
an age of 5 Myr and masses of 84+-15 MJup and 60+-25 MJup at an age of 10 Myr.
For our survey objects with spectral types later than M8, we find an upper
limit on the binary fraction of <9% (1-sigma) at separations of 10 -- 500 AU.
We combine the results of our survey with previous surveys of Upper Sco and
similar young regions to set the strongest constraints to date on binary
fraction for young substellar objects and very low mass stars. The binary
fraction for low mass (<40 MJup) brown dwarfs in Upper Sco is similar to that
for T dwarfs in the field; for higher mass brown dwarfs and very low mass
stars, there is an excess of medium-separation (10-50 AU projected separation)
young binaries with respect to the field. These medium separation binaries will
likely survive to late ages.Comment: 37 pages, 6 figures, accepted to Ap
A Spitzer Search For Planetary-Mass Brown Dwarfs With Circumstellar Disks: Candidate Selection
We report on initial results from a Spitzer program to search for very
low-mass brown dwarfs in Ophiuchus. This program is an extension of an earlier
study by Allers et al. which had resulted in an extraordinary success rate, 18
confirmed out of 19 candidates. Their program combined near-infrared and
Spitzer photom- etry to identify objects with very cool photospheres together
with circumstellar disk emission to indicate youth. Our new program has
obtained deep IRAC pho- tometry of a 0.5 deg2 field that was part of the
original Allers et al. study. We report 18 new candidates whose luminosities
extend down to 10-4 L\cdot which sug- gests masses down to ~ 2 MJ if confirmed.
We describe our selection techniques, likely contamination issues, and
follow-on photometry and spectroscopy that are in progress
A Disk Around the Planetary-Mass Companion GSC 06214-00210 b: Clues About the Formation of Gas Giants on Wide Orbits
We present Keck/OSIRIS 1.1-1.8 um adaptive optics integral field spectroscopy
of the planetary-mass companion to GSC 06214-00210, a member of the ~5 Myr
Upper Scorpius OB association. We infer a spectral type of L0+/-1, and our
spectrum exhibits multiple signs of youth. The most notable feature is
exceptionally strong PaBeta emission (EW=-11.4 +/- 0.3 A) which signals the
presence of a circumplanetary accretion disk. The luminosity of GSC 06214-00210
b combined with its age yields a model-dependent mass of 14 +/- 2 MJup, making
it the lowest-mass companion to show evidence of a disk. With a projected
separation of 320 AU, the formation of GSC 06214-00210 b and other very
low-mass companions on similarly wide orbits is unclear. One proposed mechanism
is formation at close separations followed by planet-planet scattering to much
larger orbits. Since that scenario involves a close encounter with another
massive body, which is probably destructive to circumplanetary disks, it is
unlikely that GSC 06214-00210 b underwent a scattering event in the past. This
implies that planet-planet scattering is not solely responsible for the
population of gas giants on wide orbits. More generally, the identification of
disks around young planetary companions on wide orbits offers a novel method to
constrain the formation pathway of these objects, which is otherwise
notoriously difficult to do for individual systems. We also refine the spectral
type of the primary from M1 to K7 and detect a mild (2-sigma) excess at 22 um
using WISE photometry.Comment: 25 pages, 13 figures; Accepted by Ap
Discovery of a Young L Dwarf Binary, SDSS J224953.47+004404.6AB
We report discovery of a young 0.32" L dwarf binary, SDSS J2249+0044AB, found
as the result of a Keck LGSAO imaging survey of young field brown dwarfs. Weak
K, Na, and FeH features as well as strong VO absorption in the integrated-light
J-band spectrum indicate a young age for the system. From spatially resolved
K-band spectra we determine spectral types of L3 and L5 for components A and B,
respectively. SDSS J2249+0044A is spectrally very similar to G196-3B, an L3
companion to a young M2.5 field dwarf. Thus, we adopt 100 Myr (the age estimate
of the G196-3 system) as the age of SDSS J2249+0044AB, but ages of 12-790 Myr
are possible. By comparison to G196-3B, we estimate a distance to SDSS
J2249+0044AB of 54 +- 16 pc and infer a projected separation of 17 +- 5 AU for
the binary. Comparison of the luminosities to evolutionary models at an age of
100 Myr yields masses of 0.029 and 0.022 Msun for SDSS J2249+0044A and B,
respectively. Over the possible ages of the system (12-790 Myr), the mass of
SDSS J2249+0044A could range from 0.011 to 0.070 Msun and the mass of SDSS
J2249+0044B could range from 0.009 to 0.065 Msun. Evolutionary models predict
that either component could be burning deuterium, which could result in a mass
ratio as low as 0.4, or alternatively, a reversal in the luminosities of the
binary. We find a likely proper motion companion, GSC 00568-01752, which lies
48.9" away (2600 AU) and has SDSS+2MASS colors consistent with an early M
dwarf. The photometric distance to GSC 00568-01752 is 53 +- 15 pc, in agreement
with our distance estimate for SDSS J2249+0044AB. The space motion of SDSS
J2249+0044AB shows no obvious coincidence with known young moving groups. The
unusually red near-IR colors, young age, and low masses of the binary make it
an important template for studying planetary-mass objects found by direct
imaging surveys.Comment: revised, accepted versio
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