844 research outputs found
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 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
Genetic and physical mapping of DNA replication origins in Haloferax volcanii
The halophilic archaeon Haloferax volcanii has a multireplicon genome, consisting of a main chromosome, three secondary chromosomes, and a plasmid. Genes for the initiator protein Cdc6/Orc1, which are commonly located adjacent to archaeal origins of DNA replication, are found on all replicons except plasmid pHV2. However, prediction of DNA replication origins in H. volcanii is complicated by the fact that this species has no less than 14 cdc6/orc1 genes. We have used a combination of genetic, biochemical, and bioinformatic approaches to map DNA replication origins in H. volcanii. Five autonomously replicating sequences were found adjacent to cdc6/orc1 genes and replication initiation point mapping was used to confirm that these sequences function as bidirectional DNA replication origins in vivo. Pulsed field gel analyses revealed that cdc6/orc1-associated replication origins are distributed not only on the main chromosome (2.9 Mb) but also on pHV1 (86 kb), pHV3 (442 kb), and pHV4 (690 kb) replicons. Gene inactivation studies indicate that linkage of the initiator gene to the origin is not required for replication initiation, and genetic tests with autonomously replicating plasmids suggest that the origin located on pHV1 and pHV4 may be dominant to the principal chromosomal origin. The replication origins we have identified appear to show a functional hierarchy or differential usage, which might reflect the different replication requirements of their respective chromosomes. We propose that duplication of H. volcanii replication origins was a prerequisite for the multireplicon structure of this genome, and that this might provide a means for chromosome-specific replication control under certain growth conditions. Our observations also suggest that H. volcanii is an ideal organism for studying how replication of four replicons is regulated in the context of the archaeal cell cycle. © 2007 Norais et al
Herschel/PACS View Of Disks Around Low-Mass Stars And Brown Dwarfs In The TW Hydrae Association
We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 mu m and 100 mu m and three targets at 160 mu m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M-disk proportional to M-star) observed mainly in low-mass stars may extend down to the brown dwarf regime.Natural Science Foundation of Jiangsu Province of China BK20141046Youth Qianren Program of the National Science Foundation of ChinaNational Aeronautics and Space AdministrationStrategic Priority Research Program >The Emergence of Cosmological Structures> of the Chinese Academy of Sciences XDB09000000Astronom
Investigating Atomic Details of the CaF(111) Surface with a qPlus Sensor
The (111) surface of CaF has been intensively studied with
large-amplitude frequency-modulation atomic force microscopy and atomic
contrast formation is now well understood. It has been shown that the apparent
contrast patterns obtained with a polar tip strongly depend on the tip
terminating ion and three sub-lattices of anions and cations can be imaged.
Here, we study the details of atomic contrast formation on CaF(111) with
small-amplitude force microscopy utilizing the qPlus sensor that has been shown
to provide utmost resolution at high scanning stability. Step edges resulting
from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp
structures and on flat terraces, the atomic corrugation is seen in high clarity
even for large area scans. The atomic structure is also not lost when scanning
across triple layer step edges. High resolution scans of small surface areas
yield contrast features of anion- and cation sub-lattices with unprecedented
resolution. These contrast patterns are related to previously reported
theoretical results.Comment: 18 pages, 9 Figures, presented at 7th Int Conf Noncontact AFM
Seattle, USA Sep 12-15 2004, accepted for publication in Nanotechnology,
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