148 research outputs found
Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey
We combine 131 new medium-resolution (R~2000) J-band spectra of M, L, and T
dwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97
previously published BDSS spectra to study surface-gravity-sensitive indices
for 228 low-mass stars and brown dwarfs spanning spectral types M5-T9.
Specifically, we use an established set of spectral indices to determine
surface gravity classifications for all M6-L7 objects in our sample by
measuring equivalent widths (EW) of the K I lines at 1.1692, 1.1778, 1.2529 um,
and the 1.2 um FeHJ absorption index. Our results are consistent with previous
surface gravity measurements, showing a distinct double peak - at ~L5 and T5 -
in K I EW as a function of spectral type. We analyze K I EWs of 73 objects of
known ages and find a linear trend between log(Age) and EW. From this
relationship, we assign age ranges to the very low gravity, intermediate
gravity, and field gravity designations for spectral types M6-L0.
Interestingly, the ages probed by these designations remain broad, change with
spectral type, and depend on the gravity sensitive index used. Gravity
designations are useful indicators of the possibility of youth, but current
datasets cannot be used to provide a precise age estimate.Comment: 33 pages, 13 figures, ApJ in pres
Microbial Selection and Survival in Subseafloor Sediment
Many studies have examined relationships of microorganisms to geochemical zones in subseafloor sediment. However, responses to selective pressure and patterns of community succession with sediment depth have rarely been examined. Here we use 16S rDNA sequencing to examine the succession of microbial communities at sites in the Indian Ocean and the Bering Sea. The sediment ranges in depth from 0.16 to 332 m below seafloor and in age from 660 to 1,300,000 years. The majority of subseafloor taxonomic diversity is present in the shallowest depth sampled. The best predictor of sequence presence or absence in the oldest sediment is relative abundance in the near-seafloor sediment. This relationship suggests that perseverance of specific taxa into deep, old sediment is primarily controlled by the taxonomic abundance that existed when the sediment was near the seafloor. The operational taxonomic units that dominate at depth comprise a subset of the local seafloor community at each site, rather than a grown-in group of geographically widespread subseafloor specialists. At both sites, most taxa classified as abundant decrease in relative frequency with increasing sediment depth and age. Comparison of community composition to cell counts at the Bering Sea site indicates that the rise of the few dominant taxa in the deep subseafloor community does not require net replication, but might simply result from lower mortality relative to competing taxa on the long timescale of community burial
Fossil DNA persistence and decay in marine sediment over hundred-thousand-year to million-year time scales
DNA in marine sediment contains both fossil sequences and sequences from organisms that live in the sediment. The demarcation between these two pools and their respective rates of turnover are generally unknown. We address these issues by comparing the total extractable DNA pool to the fraction of sequenced chloroplast DNA (cpDNA) in sediment from two sites in the Bering Sea. We assume that cpDNA is a tracer of non-reproducing fossil DNA. Given \u3e150,000 sequence reads per sample, cpDNA is easily detectable in the shallowest samples but decays with depth, suggesting that sequencing-based richness assessments of communities in deep subseafloor sediment are relatively unaffected by fossil DNA. The initial decrease in cpDNA reads suggests that most cpDNA decays within 100â200 k.y. of deposition. However, cpDNA from a few phylotypes, including some that match fossil diatoms, are present throughout the cored sediment, ranging in age to 1.4 Ma. The relative fraction of sequences composed by cpDNA decreases non-linearly with increasing sediment age, suggesting that detectable cpDNA becomes more recalcitrant with age. This can be explained by biological activity decreasing with sediment age and/or by preferential long-term survival of only the most thoroughly protected DNA. The association of cpDNA reads with published records of siliceous microfossils, including diatom spores, at the same sites suggests that microfossils may help to preserve DNA. This DNA may be useful for studies of paleoenvironmental conditions and biological evolution on time scales that approach or exceed 1 m.y
Microbial Selection and Survival in Subseafloor Sediment
Many studies have examined relationships of microorganisms to geochemical zones in subseafloor sediment. However, responses to selective pressure and patterns of community succession with sediment depth have rarely been examined. Here we use 16S rDNA sequencing to examine the succession of microbial communities at sites in the Indian Ocean and the Bering Sea. The sediment ranges in depth from 0.16 to 332 m below seafloor and in age from 660 to 1,300,000 years. The majority of subseafloor taxonomic diversity is present in the shallowest depth sampled. The best predictor of sequence presence or absence in the oldest sediment is relative abundance in the near-seafloor sediment. This relationship suggests that perseverance of specific taxa into deep, old sediment is primarily controlled by the taxonomic abundance that existed when the sediment was near the seafloor. The operational taxonomic units that dominate at depth comprise a subset of the local seafloor community at each site, rather than a grown-in group of geographically widespread subseafloor specialists. At both sites, most taxa classified as abundant decrease in relative frequency with increasing sediment depth and age. Comparison of community composition to cell counts at the Bering Sea site indicates that the rise of the few dominant taxa in the deep subseafloor community does not require net replication, but might simply result from lower mortality relative to competing taxa on the long timescale of community burial
Determining the Physical Properties of Very-Low-Mass Stars and Brown Dwarfs in the Near-Infrared
Accurate measurements of the fundamental physical properties of veryâlowâmass stars and brown dwarfs are crucial for calibrating evolutionary models. Photometry and lowâresolution spectroscopy effectively average over absorption features that sample different layers in complex cool atmospheres. By studying a large sample of objects bright enough for highâresolution spectroscopy, we can develop methods for determining physical properties as accurately and efficiently as possible. As part of the Brown Dwarf Spectroscopic Survey (BDSS; [1, 2]), we are conducting a detailed comparison of observed and synthetic spectra for a sample of young M and L dwarfs and field M, L, and T dwarfs ( âŒ50 objects in total). Highâresolution nearâinfrared spectra from NIRSPEC on Keck II provide an unequaled combination of resolving power and wavelength coverage. Synthetic spectra were created from PHOENIX atmosphere models calculated exclusively for this project with updated line lists and solar abundances. Combined with spectral types from photometric studies and lowâresolution spectra and surface gravity estimates from age determination, the highâresolution spectra enable precise measurements of effective temperature and surface gravity, as well as accurate determination of radial velocity and projected rotational velocity. Our preliminary observationâmodel comparisons distinguish between wavelength regimes for which the models reproduce observed highâresolution spectra and regimes in which model data (line lists, oscillator strengths, etc.) are lacking
The NIRSPEC Brown Dwarf Spectroscopic Survey II: High-Resolution J-Band Spectra of M, L and T Dwarfs
We present a sequence of high resolution (R~20,000 or 15 km/s) infrared
spectra of stars and brown dwarfs spanning spectral types M2.5 to T6.
Observations of 16 objects were obtained using eight echelle orders to cover
part of the J-band from 1.165-1.323 micron with NIRSPEC on the Keck II
telescope. By comparing opacity plots and line lists, over 200 weak features in
the J-band are identified with either FeH or H2O transitions. Absorption by FeH
attains maximum strength in the mid-L dwarfs, while H2O absorption becomes
systematically stronger towards later spectral types. Narrow resolved features
broaden markedly after the M to L transition. Our high resolution spectra also
reveal that the disappearance of neutral Al lines at the boundary between M and
L dwarfs is remarkably abrupt, presumably because of the formation of grains.
Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed by
condensation. The neutral potassium (K I) doublets that dominate the J-band
have pressure broadened wings that continue to broaden from ~50 km/s (FWHM) at
mid-M to ~500 km/s at mid-T. In contrast however, the measured
pseudo-equivalent widths of these same lines reach a maximum in the mid-L
dwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines without
extensive pressure-broadened wings, indicative of a lower gravity atmosphere.
Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH and
H2O features, confirming its status as a rapid rotator. In contrast to other
late T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absence
of potassium even at high resolution, which may be a metallicity effect or a
result of a cooler, higher-gravity atmosphere.Comment: 53 pages, 21 figures, data will be available at
http://www.astro.ucla.edu/~mclean/BDSSarchive
2MASSJ035523.51+113337.4: A Young, Dusty, Nearby, Isolated Brown Dwarf Resembling A Giant Exoplanet
We present parallax and proper motion measurements, near-infrared spectra,
and WISE photometry for the low surface gravity L5gamma dwarf
2MASSJ035523.37+113343.7 (2M0355). We use these data to evaluate photometric,
spectral, and kinematic signatures of youth as 2M0355 is the reddest isolated L
dwarf yet classified. We confirm its low-gravity spectral morphology and find a
strong resemblance to the sharp triangular shaped -band spectrum of the 10
Myr planetary-mass object 2M1207b. We find that 2M0355 is underluminous
compared to a normal field L5 dwarf in the optical and MKO J,H, and K bands and
transitions to being overluminous from 3-12 microns, indicating that enhanced
photospheric dust shifts flux to longer wavelengths for young, low-gravity
objects, creating a red spectral energy distribution. Investigating the
near-infrared color magnitude diagram for brown dwarfs confirms that 2M0355 is
redder and underluminous compared to the known brown dwarf population, similar
to the peculiarities of directly imaged exoplanets 2M1207b and HR8799bcd. We
calculate UVW space velocities and find that the motion of 2M0355 is consistent
with young disk objects (< 2-3 Gyr) and it shows a high likelihood of
membership in the AB Doradus association.Comment: 23 pages, 10 figures, 5 Tables, Submitted to AJ 13 May 201
Physical Properties of Young Brown Dwarfs and Very Low-Mass Stars Inferred from High-Resolution Model Spectra
By comparing near-infrared spectra with atmosphere models, we infer the
effective temperature, surface gravity, projected rotational velocity, and
radial velocity for 21 very-low-mass stars and brown dwarfs. The unique sample
consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects
and one of >1 Gyr field objects. A third sequence is comprised of only ~M6
objects with ages ranging from 1 Gyr. Spectra were obtained in the J
band at medium (R~2,000) and high (R~20,000) resolutions with NIRSPEC on the
Keck II telescope. Synthetic spectra were generated from atmospheric structures
calculated with the PHOENIX model atmosphere code. Using multi-dimensional
least-squares fitting and Monte Carlo routines we determine the best-fit model
parameters for each observed spectrum and note which spectral regions provide
consistent results. We identify successes in the reproduction of observed
features by atmospheric models, including pressure-broadened KI lines, and
investigate deficiencies in the models, particularly missing FeH opacity, that
will need to be addressed in order to extend our analysis to cooler objects.
The precision that can be obtained for each parameter using medium- and high-
resolution near-infrared spectra is estimated and the implications for future
studies of very low mass stars and brown dwarfs are discussed.Comment: Accepted to the Astrophysical Journal Supplement Serie
Discoveries from a Near-infrared Proper Motion Survey using Multi-epoch 2MASS Data
We have conducted a 4030-square-deg near-infrared proper motion survey using
multi-epoch data from the Two Micron All-Sky Survey (2MASS). We find 2778
proper motion candidates, 647 of which are not listed in SIMBAD. After
comparison to DSS images, we find that 107 of our proper motion candidates lack
counterparts at B-, R-, and I-bands and are thus 2MASS-only detections. We
present results of spectroscopic follow-up of 188 targets that include the
infrared-only sources along with selected optical-counterpart sources with
faint reduced proper motions or interesting colors. We also establish a set of
near-infrared spectroscopic standards with which to anchor near-infrared
classifications for our objects. Among the discoveries are six young field
brown dwarfs, five "red L" dwarfs, three L-type subdwarfs, twelve M-type
subdwarfs, eight "blue L" dwarfs, and several T dwarfs. We further refine the
definitions of these exotic classes to aid future identification of similar
objects. We examine their kinematics and find that both the "blue L" and "red
L" dwarfs appear to be drawn from a relatively old population. This survey
provides a glimpse of the kinds of research that will be possible through
time-domain infrared projects such as the UKIDSS Large Area Survey, various
VISTA surveys, and WISE, and also through z- or y-band enabled, multi-epoch
surveys such as Pan-STARRS and LSST.Comment: To appear in the September 2010 issue of The Astrophysical Journal,
Supplement Serie
Relationship of bacterial richness to organic degradation rate and sediment age in subseafloor sediment
© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Applied and Environmental Microbiology 82 (2016): 4994-4999, doi:10.1128/AEM.00809-16.Subseafloor sediment hosts a large, taxonomically rich and metabolically diverse microbial ecosystem. However, the factors that control microbial diversity in subseafloor sediment have rarely been explored. Here we show that bacterial richness varies with organic degradation rate and sediment age. At three open-ocean sites (in the Bering Sea and equatorial Pacific) and one continental margin site (Indian Ocean), richness decreases exponentially with increasing sediment depth. The rate of decrease in richness with depth varies from site to site. The vertical succession of predominant terminal electron acceptors correlates to abundance-weighted community composition, but does not drive the vertical decrease in richness. Vertical patterns of richness at the open-ocean sites closely match organic degradation rates; both properties are highest near the seafloor and decline together as sediment depth increases. This relationship suggests that (i) total catabolic activity and/or electron donor diversity exerts a primary influence on bacterial richness in marine sediment, and (ii) many bacterial taxa that are poorly adapted for subseafloor sedimentary conditions are degraded in the geologically young sediment where respiration rates are high. Richness consistently takes a few hundred thousand years to decline from near-seafloor values to much lower values in deep anoxic subseafloor sediment, regardless of sedimentation rate, predominant terminal electron acceptor, or oceanographic context.This work, including the efforts of Mitchell L. Sogin and Steven DâHondt,
was funded by Sloan Foundation (Census of Deep Life). This work, including
the efforts of Steven DâHondt, was funded by U.S. Science Support
Program for IODP. This work, including the efforts of Steven
DâHondt, was funded by National Science Foundation (NSF) (OCE-
0752336 and OCE-0939564).
The work of E. A. Walsh, J. B. Kirkpatrick, R. Pockalny, and J. Sauvage was
funded by the grants to S. DâHondt
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