203 research outputs found
Laboratory Determination of the Infrared Band Strengths of Pyrene Frozen in Water Ice: Implications for the Composition of Interstellar Ices
Broad infrared emission features (e.g., at 3.3, 6.2, 7.7, 8.6, and 11.3
microns) from the gas phase interstellar medium have long been attributed to
polycyclic aromatic hydrocarbons (PAHs). A significant portion (10%-20%) of the
Milky Way's carbon reservoir is locked in PAH molecules, which makes their
characterization integral to our understanding of astrochemistry. In molecular
clouds and the dense envelopes and disks of young stellar objects (YSOs), PAHs
are expected to be frozen in the icy mantles of dust grains where they should
reveal themselves through infrared absorption. To facilitate the search for
frozen interstellar PAHs, laboratory experiments were conducted to determine
the positions and strengths of the bands of pyrene mixed with H2O and D2O ices.
The D2O mixtures are used to measure pyrene bands that are masked by the strong
bands of H2O, leading to the first laboratory determination of the band
strength for the CH stretching mode of pyrene in water ice near 3.25 microns.
Our infrared band strengths were normalized to experimentally determined
ultraviolet band strengths, and we find that they are generally ~50% larger
than those reported by Bouwman et al. based on theoretical strengths. These
improved band strengths were used to reexamine YSO spectra published by Boogert
et al. to estimate the contribution of frozen PAHs to absorption in the 5-8
micron spectral region, taking into account the strength of the 3.25 micron CH
stretching mode. It is found that frozen neutral PAHs contain 5%-9% of the
cosmic carbon budget, and account for 2%-9% of the unidentified absorption in
the 5-8 micron region.Comment: Accepted for publication in ApJ on 14 Feb 201
Scaling K2. I. Revised Parameters for 222,088 K2 Stars and a K2 Planet Radius Valley at 1.9 R_ā
Previous measurements of stellar properties for K2 stars in the Ecliptic Plane Input Catalog largely relied on photometry and proper motion measurements, with some added information from available spectra and parallaxes. Combining Gaia DR2 distances with spectroscopic measurements of effective temperatures, surface gravities, and metallicities from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) DR5, we computed updated stellar radii and masses for 26,838 K2 stars. For 195,250 targets without a LAMOST spectrum, we derived stellar parameters using random forest regression on photometric colors trained on the LAMOST sample. In total, we measured spectral types, effective temperatures, surface gravities, metallicities, radii, and masses for 222,088 A, F, G, K, and M-type K2 stars. With these new stellar radii, we performed a simple reanalysis of 299 confirmed and 517 candidate K2 planet radii from Campaigns 1ā13, elucidating a distinct planet radius valley around 1.9 R_ā, a feature thus far only conclusively identified with Kepler planets, and tentatively identified with K2 planets. These updated stellar parameters are a crucial step in the process toward computing K2 planet occurrence rates
Genetic Structure of Halodule wrightii Populations from the Laguna Madre Region in the Western Gulf of Mexico
A random amplified polymorphic DNA assay was used to assess genetic variation in populations of the seagrass Halodule wrightii (Ascherson) from the western Gulf of Mexico. This region includes one of the world\u27s few hypersaline lagoons (Laguna Madre) and contains the vast majority of seagrasses on the Texas coast. Results indicate a moderate amount of genetic diversity among populations. The highest level (Hc= 0.33) was found in a population from Nueces Bay, a disturbed site in the Coastal Bend area, whereas the lowest was found in a Lower Laguna Madre population (Hc = 0.15). Genetic differentiation generally followed an isolation-by-distance model. The Nueces Bay population also showed the greatest degree of differentiation, whereas the Redflsh Bay and Lower Laguna Madre populations were relatively similar (ĻST = 0.091). Combined with previous results, we now have a rudimentary picture of genetic variation in this species from the Texas Gulf Coast
Medium-separation binaries do not affect the first steps of planet formation
The first steps of planet formation are marked by the growth and
crystallization of sub-micrometer-sized dust grains accompanied by dust
settling toward the disk midplane. In this paper we explore whether the first
steps of planet formation are affected by the presence of medium-separation
stellar companions. We selected two large samples of disks around single and
binary T Tauri stars in Taurus that are thought to have only a modest age
spread of a few Myr. The companions of our binary sample are at projected
separations between 10 and 450 AU with masses down to about 0.1 solar masses.
We used the strength and shape of the 10 micron silicate emission feature as a
proxy for grain growth and for crystallization respectively. The degree of dust
settling was evaluated from the ratio of fluxes at two different mid-infrared
wavelengths. We find no statistically significant difference between the
distribution of 10 micron silicate emission features from single and binary
systems. In addition, the distribution of disk flaring is indistinguishable
between the single and binary system samples. These results show that the first
steps of planet formation are not affected by the presence of a companion at
tens of AU.Comment: To appear in the Astrophysical Journa
Observed Variability at 1um and 4um in the Y0 Brown Dwarf WISEP J173835.52+273258.9
We have monitored photometrically the Y0 brown dwarf WISEP
J173835.52+273258.9 (W1738) at both near- and mid-infrared wavelengths. This ~1
Gyr-old 400K dwarf is at a distance of 8pc and has a mass around 5 M_Jupiter.
We observed W1738 using two near-infrared filters at lambda~1um, Y and J, on
Gemini observatory, and two mid-infrared filters at lambda~4um, [3.6] and
[4.5], on the Spitzer observatory. Twenty-four hours were spent on the source
by Spitzer on each of June 30 and October 30 2013 UT. Between these
observations, around 5 hours were spent on the source by Gemini on each of July
17 and August 23 2013 UT. The mid-infrared light curves show significant
evolution between the two observations separated by four months. We find that a
double sinusoid can be fit to the [4.5] data, where one sinusoid has a period
of 6.0 +/- 0.1 hours and the other a period of 3.0 +/- 0.1 hours. The
near-infrared observations suggest variability with a ~3.0 hour period,
although only at a <~2 sigma confidence level. We interpret our results as
showing that the Y dwarf has a 6.0 +/- 0.1 hour rotation period, with one or
more large-scale surface features being the source of variability. The
peak-to-peak amplitude of the light curve at [4.5] is 3%. The amplitude of the
near-infrared variability, if real, may be as high as 5 to 30%. Intriguingly,
this size of variability and the wavelength dependence can be reproduced by
atmospheric models that include patchy KCl and Na_2S clouds and associated
small changes in surface temperature. The small number of large features, and
the timescale for evolution of the features, is very similar to what is seen in
the atmospheres of the solar system gas giants.Comment: Accepted by ApJ July 26 2016. Twenty-six pages include 8 Figures and
5 Table
Hydrologic and Erosion Responses of Sagebrush Steppe Following Juniper Encroachment, Wildfire, and Tree Cutting
Extensive woodland expansion in the Great Basin has generated concern regarding ecological impacts of tree encroachment on sagebrush rangelands and strategies for restoring sagebrush steppe. This study used rainfall (0.5 m2 and 13 m2 scales) and concentrated flow simulations and measures of vegetation, ground cover, and soils to investigate hydrologic and erosion impacts of western juniper (Juniperus occidentalis Hook.) encroachment into sagebrush steppe and to evaluate short-term effects of burning and tree cutting on runoff and erosion responses. The overall effects of tree encroachment were a reduction in understory vegetation and formation of highly erodible, bare intercanopy between trees. Runoff and erosion from high-intensity rainfall (102 mmāĀ·āhā1, 13 m2 plots) were generally low from unburned areas underneath tree canopies (13 mm and 48 gāĀ·āmā2) and were higher from the unburned intercanopy (43 mm and 272 gāĀ·āmā2). Intercanopy erosion increased linearly with runoff and exponentially where bare ground exceeded 60%. Erosion from simulated concentrated flow was 15- to 25-fold greater from the unburned intercanopy than unburned tree canopy areas. Severe burning amplified erosion from tree canopy plots by a factor of 20 but had a favorable effect on concentrated flow erosion from the intercanopy. Two years postfire, erosion remained 20-fold greater on burned than unburned tree plots, but concentrated flow erosion from the intercanopy (76% of study area) was reduced by herbaceous recruitment. The results indicate burning may amplify runoff and erosion immediately postfire. However, we infer burning that sustains residual understory cover and stimulates vegetation productivity may provide long-term reduction of soil loss relative to woodland persistence. Simply placing cut-downed trees into the unburned intercanopy had minimal immediate impact on infiltration and soil loss. Results suggest cut-tree treatments should focus on establishing tree debris contact with the soil surface if treatments are expected to reduce short-term soil loss during the postcut understory recruitment period
- ā¦