424 research outputs found
Evidence for Variable, Correlated X-ray and Optical/IR Extinction toward the Nearby, Pre-main Sequence Binary TWA 30
We present contemporaneous XMM-Newton X-ray and ground-based optical/near-IR
spectroscopic observations of the nearby ( pc), low-mass (mid-M)
binary system TWA 30A and 30B. The components of this wide (separation
3400 AU) binary are notable for their nearly edge-on disk viewing
geometries, high levels of variability, and evidence for collimated stellar
outflows. We obtained XMM-Newton X-ray observations of TWA 30A and 30B in 2011
June and July, accompanied (respectively) by IRTF SpeX (near-IR) and VLT
XSHOOTER (visible/near-IR) spectroscopy obtained within 20 hours of the
X-ray observations. TWA 30A was detected in both XMM-Newton observations at
relatively faint intrinsic X-ray luminosities (
) compared to stars of similar mass and age . The intrinsic
(0.15-2.0 keV) X-ray luminosities measured in 2011 had decreased by a factor
20-100 relative to a 1990 (ROSAT) X-ray detection. TWA 30B was not detected,
and we infer an upper limit of ( 3.0 erg
s). We measured a large change in visual extinction toward TWA 30A (from
to ) between the two 2011 observing epochs,
and we find evidence for a corresponding significant decrease in X-ray
absorbing column (). The apparent correlated change in and is
suggestive of variable obscuration of the stellar photosphere by disk material
composed of both gas and dust. However, in both observations, the inferred
to ratio is lower than that typical of the ISM, suggesting that
the disk is either depleted of gas or is deficient in metals in the gas phase.Comment: 10 pages, 7 figures, Accepted for publication in MNRA
Genetic structure and genetic diversity of Swietenia macrophylla (Meliaceae): implications for sustainable forest management in Mexico
The genetic structure and genetic diversity of Mexican populations of Swietenia macrophylla were evaluated. In Mexico, this neotropical tree reaches its northernmost distribution limit. The relationship between genetic diversity and geographic position (longitude, latitude) of four populations was described. The mean genetic and pairwise genetic differentiations were estimated to study the geographic pattern in the genetic structure. The mean overall values for observed heterozygosis (Ho), expected heterozygosis (He) and fixation coefficient (Fis) were 0.41, 0.71 and 0.449 respectively. The mean values per population of Ho, He and Fis correlated with latitude only. The genetic differentiation between populations revealed by the coefficient of differentiation (Fst= 0.047) was statistically significant (p=0.0001). Unweighted pair group method analysis showed that the extent of pairwise genetic differentiation increased with latitudinal position of populations, although no isolation by distance was observed (r=0.76, p=0.15). Results were discussed in the context of the marginal distribution of Mexican populations. Implications of the results on the conservation of S. macrophylla , particularly for populations located in the Mayan zone were discussed
The ultracool dwarf DENIS-P J104814.7-395606. Chromospheres and coronae at the low-mass end of the main-sequence
We have obtained an XMM-Newton observation and a broad-band spectrum from the
ultraviolet to the near infrared with X-Shooter for one of the nearest M9
dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of
activity parameters for ultracool dwarfs from the literature with the aim to
advance our understanding of these objects by comparing them to early-M type
dwarf stars and the Sun.
Our deep XMM-Newton observation has led to the first X-ray detection of
DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V
band brightness (V = 17.35mag). Flux-flux relations between X-ray and
chromospheric activity indicators are here for the first time extended into the
regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956
and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests
that the same heating mechanisms work in the atmospheres of ultracool dwarfs,
albeit weaker as judged from their lower fluxes. The observed Balmer decrements
of DENIS-P J1048-3956 are compatible with optically thick plasma in LTE at low,
nearly photospheric temperature or optically thin LTE plasma at 20000K.
Describing the decrements with CaseB recombination requires different emitting
regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta
flux ratio is also poorly fitted by the optically thin models. We derive a
similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and
conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P
J1048-3956 to the list of ultracool dwarfs detected in both the radio and the
X-ray band. The Benz-Guedel relation between radio and X-ray luminosity of
late-type stars is well-known to be violated by ultracool dwarfs. We speculate
on the presence of two types of ultracool dwarfs with distinct radio and X-ray
behavior.Comment: accepted for publication in Astronomy & Astrophysic
A study of the selective hydroconversion of biocarboxylic acids to bioalcohols over novel indium-nickel/zeolite catalysts using octanoic acid as model reactant
Octanoic acid (OA) was hydrotreated in a flow-through reactor at 21 bar total pressure and 240-340 °C over supported metal catalysts prepared from Ni-zeolites (A, X, P) by indium modification. The Ni-zeolites were activated first in H2 flow at 21 bar and 450 °C. While a fraction of the nickel got fully reduced, the zeolite structure became partially destructed. However, some nickel cations remained unreduced, therefore a large fraction of the crystalline zeolite structure was retained. The indium modification of the reduced Ni-zeolites generated bimetallic NiIn/Ni,H-zeolite catalysts having higher stability, hydroconversion activity, octanol selectivity, and lower hydrodecarbonylation activity than the parent partially destructed Ni-zeolite and the Adkins-type catalysts, commercially applied for the conversion of fatty acids to alcohols. © 2013 Akadémiai Kiadó, Budapest, Hungary
A Search for Very Low-mass Stars and Brown Dwarfs in the Young sigma Orionis Cluster
We present a CCD-based photometric survey covering 870 sq. arcmin in a young
stellar cluster around the young multiple star sigma Orionis. Our survey
limiting R, I, and Z magnitudes are 23.2, 21.8, and 21.0, respectively. From
our colour-magnitude diagrams, we have selected 49 faint objects, which
smoothly extrapolate the photometric sequence defined by more massive known
members. Adopting the currently accepted age interval of 2-10 Myr for the Orion
1b association and considering recent evolutionary models, our objects may span
a mass range from 0.1 down to 0.02 Msun, well within the substellar regime.
Follow-up low-resolution optical spectroscopy (635-920 nm) for eight of our
candidates (I=16-19.5) shows that they have spectral types M6-M8.5 which are
consistent with the expectations for true members. Compared with their Pleiades
counterparts of similar types, Halpha emission is generally stronger, while NaI
and KI absorption lines appear weaker, as expected for lower surface gravities
and younger ages. Additionally, TiO bands and in particular VO bands appear
clearly enhanced in our candidate with the latest spectral type, SOri 45 (M8.5,
I=19.5), compared to objects of similar types in older clusters and the field.
We have estimated the mass of this candidate at only 0.020-0.040 Msun, hence it
is one of the least massive brown dwarfs yet discovered. We also discuss in
this paper the potential role of deuterium as a tracer of both substellar
nature and age in very young clusters.Comment: Accepted for publication in ApJ Main Journal. 32 pages of text and
tables + 9 pages of figures. Figures 3a and 3b (gif format) provided
separatel
Effect of unfolding on the tryptophanyl fluorescence lifetime distribution in apomyoglobin
Insights from Amphioxus into the Evolution of Vertebrate Cartilage
Central to the story of vertebrate evolution is the origin of the vertebrate head, a problem difficult to approach using paleontology and comparative morphology due to a lack of unambiguous intermediate forms. Embryologically, much of the vertebrate head is derived from two ectodermal tissues, the neural crest and cranial placodes. Recent work in protochordates suggests the first chordates possessed migratory neural tube cells with some features of neural crest cells. However, it is unclear how and when these cells acquired the ability to form cellular cartilage, a cell type unique to vertebrates. It has been variously proposed that the neural crest acquired chondrogenic ability by recruiting proto-chondrogenic gene programs deployed in the neural tube, pharynx, and notochord. To test these hypotheses we examined the expression of 11 amphioxus orthologs of genes involved in neural crest chondrogenesis. Consistent with cellular cartilage as a vertebrate novelty, we find that no single amphioxus tissue co-expresses all or most of these genes. However, most are variously co-expressed in mesodermal derivatives. Our results suggest that neural crest-derived cartilage evolved by serial cooption of genes which functioned primitively in mesoderm
C2D Spitzer-IRS spectra of disks around T Tauri stars: I. Silicate emission and grain growth
Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7
intermediate-mass Herbig Ae stars with circumstellar disks were obtained using
the Spitzer Space Telescope as part of the c2d IRS survey. This work
complements prior spectroscopic studies of silicate infrared emission from
disks, which were focused on intermediate-mass stars, with observations of
solar-mass stars limited primarily to the 10 um region. The observed 10 and 20
um silicate feature strengths/shapes are consistent with source-to-source
variations in grain size. A large fraction of the features are weak and flat,
consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um
in radius). In addition, approximately half of the T Tauri star spectra show
crystalline silicate features near 28 and 33 um indicating significant
processing when compared to interstellar grains. A few sources show large
10-to-20 um ratios and require even larger grains emitting at 20 um than at 10
um. This size difference may arise from the difference in the depth into the
disk probed by the two silicate emission bands in disks where dust settling has
occurred. The 10 um feature strength vs. shape trend is not correlated with age
or Halpha equivalent width, suggesting that some amount of turbulent mixing and
regeneration of small grains is occurring. The strength vs. shape trend is
related to spectral type, however, with M stars showing significantly flatter
10 um features (larger grain sizes) than A/B stars. The connection between
spectral type and grain size is interpreted in terms of the variation in the
silicate emission radius as a function of stellar luminosity, but could also be
indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation,
stellar/disk winds, etc.).Comment: 17 pages, 13 figures, accepted for publication by ApJ, formatted with
emulateapj using revtex4 v4.
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