80 research outputs found
Transitional disks and their origins: an infrared spectroscopic survey of Orion A
Transitional disks are protoplanetary disks around young stars, with inner
holes or gaps which are surrounded by optically thick outer, and often inner,
disks. Here we present observations of 62 new transitional disks in the Orion A
star-forming region. These were identified using the \textit{Spitzer Space
Telescope}'s Infrared Spectrograph and followed up with determinations of
stellar and accretion parameters using the Infrared Telescope Facility's SpeX.
We combine these new observations with our previous results on transitional
disks in Taurus, Chamaeleon I, Ophiuchus and Perseus, and with archival X-ray
observations. This produces a sample of 105 transitional disks of "cluster" age
3 Myr or less, by far the largest hitherto assembled. We use this sample to
search for trends between the radial structure in the disks and many other
system properties, in order to place constraints on the possible origins of
transitional disks. We see a clear progression of host star accretion rate and
the different disk morphologies. We confirm that transitional disks with
complete central clearings have median accretion rates an order of magnitude
smaller than radially continuous disks of the same population. Pre-transitional
disks --- those objects with gaps that separate inner and outer disks --- have
median accretion rates intermediate between the two. Our results from the
search for statistically significant trends, especially related to ,
strongly support that in both cases the gaps are far more likely to be due to
the gravitational influence of Jovian planets or brown dwarfs orbiting within
the gaps, than to any of the photoevaporative, turbulent or grain-growth
processes that can lead to disk dissipation. We also find that the fraction of
Class II YSOs which are transitional disks is large, 0.1-0.2, especially in the
youngest associations.Comment: 96 pages, 25 figures, resubmitted to Ap
Spitzer observations of HH54 and HH7-11: mapping the H2 ortho-to-para ratio in shocked molecular gas
We report the results of spectroscopic mapping observations carried out
toward the Herbig-Haro objects HH7-11 and HH54 over the 5.2 - 37 micron region
using the Infrared Spectrograph of the Spitzer Space Telescope. These
observations have led to the detection and mapping of the S(0) - S(7) pure
rotational lines of molecular hydrogen, together with emissions in fine
structure transitions of Ne+, Si+, S, and Fe+. The H2 rotational emissions
indicate the presence of warm gas with a mixture of temperatures in the range
400 - 1200 K, consistent with the expected temperature behind nondissociative
shocks of velocity ~ 10 - 20 km/s, while the fine structure emissions originate
in faster shocks of velocity 35 - 90 km/s that are dissociative and ionizing.
Maps of the H2 line ratios reveal little spatial variation in the typical
admixture of gas temperatures in the mapped regions, but show that the H2
ortho-to-para ratio is quite variable, typically falling substantially below
the equilibrium value of 3 attained at the measured gas temperatures. The
non-equilibrium ortho-to-para ratios are characteristic of temperatures as low
as ~ 50 K, and are a remnant of an earlier epoch, before the gas temperature
was elevated by the passage of a shock. Correlations between the gas
temperature and H2 ortho-to-para ratio show that ortho-to-para ratios < 0.8 are
attained only at gas temperatures below ~ 900 K; this behavior is consistent
with theoretical models in which the conversion of para- to ortho-H2 behind the
shock is driven by reactive collisions with atomic hydrogen, a process which
possesses a substantial activation energy barrier (E_A/k ~ 4000 K) and is
therefore very inefficient at low temperature.Comment: 45 pages, including 16 figures. Accepted for publication in Ap
Optical mapping discerns genome wide DNA methylation profiles
BACKGROUND: Methylation of CpG dinucleotides is a fundamental mechanism of epigenetic regulation in eukaryotic genomes. Development of methods for rapid genome wide methylation profiling will greatly facilitate both hypothesis and discovery driven research in the field of epigenetics. In this regard, a single molecule approach to methylation profiling offers several unique advantages that include elimination of chemical DNA modification steps and PCR amplification. RESULTS: A single molecule approach is presented for the discernment of methylation profiles, based on optical mapping. We report results from a series of pilot studies demonstrating the capabilities of optical mapping as a platform for methylation profiling of whole genomes. Optical mapping was used to discern the methylation profile from both an engineered and wild type Escherichia coli. Furthermore, the methylation status of selected loci within the genome of human embryonic stem cells was profiled using optical mapping. CONCLUSION: The optical mapping platform effectively detects DNA methylation patterns. Due to single molecule detection, optical mapping offers significant advantages over other technologies. This advantage stems from obviation of DNA modification steps, such as bisulfite treatment, and the ability of the platform to assay repeat dense regions within mammalian genomes inaccessible to techniques using array-hybridization technologies
Optical nanofibers and spectroscopy
We review our recent progress in the production and characterization of
tapered optical fibers with a sub-wavelength diameter waist. Such fibers
exhibit a pronounced evanescent field and are therefore a useful tool for
highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist
or of the medium surrounding. We use a carefully designed flame pulling process
that allows us to realize preset fiber diameter profiles. In order to determine
the waist diameter and to verify the fiber profile, we employ scanning electron
microscope measurements and a novel accurate in situ optical method based on
harmonic generation. We use our fibers for linear and non-linear absorption and
fluorescence spectroscopy of surface-adsorbed organic molecules and investigate
their agglomeration dynamics. Furthermore, we apply our spectroscopic method to
quantum dots on the surface of the fiber waist and to caesium vapor surrounding
the fiber. Finally, towards dispersive measurements, we present our first
results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B.
Changes according to referee suggestions: changed title, clarification of
some points in the text, added references, replacement of Figure 13
Best practices for measuring emerging light-emitting diode technologies
The arrival of light-emitting diodes based on new materials is posing challenges for the characterization and comparison of devices in a trusted and consistent manner. Here we provide some advice and guidelines that we hope will benefit the community
Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region
Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust
at two temperatures to probe the radial variation in dust composition in the
uppermost layers of protoplanetary disks. Most spectra indicating crystalline
silicates require Mg-rich minerals and silica, but a few suggest otherwise.
Spectra indicating abundant enstatite at higher temperatures also require
crystalline silicates at temperatures lower than those required for spectra
showing high abundance of other crystalline silicates. A few spectra show 10
micron complexes of very small equivalent width. They are fit well using
abundant crystalline silicates but very few large grains, inconsistent with the
expectation that low peak-to-continuum ratio of the 10 micron complex always
indicates grain growth. Most spectra in our sample are fit well without using
the opacities of large crystalline silicate grains. If large grains grow by
agglomeration of submicron grains of all dust types, the amorphous silicate
components of these aggregates must typically be more abundant than the
crystalline silicate components. Crystalline silicate abundances correlate
positively with other such abundances, suggesting that crystalline silicates
are processed directly from amorphous silicates and that neither forsterite,
enstatite, nor silica are intermediate steps when producing either of the other
two. Disks with more dust settling typically have greater crystalline
abundances. Large-grain abundance is somewhat correlated with greater settling
of disks. The lack of strong correlation is interpreted to mean that settling
of large grains is sensitive to individual disk properties. Lower-mass stars
have higher abundances of large grains in their inner regions.Comment: 84 pages, 27 figures, submitted to the Astrophysical Journal on 7
November, 200
A Spitzer IRS Survey of NGC 1333: Insights into disk evolution from a very young cluster
We report on the {\lambda} = 5-36{\mu}m Spitzer Infrared Spectrograph spectra
of 79 young stellar objects in the very young nearby cluster NGC 1333. NGC
1333's youth enables the study of early protoplanetary disk properties, such as
the degree of settling as well as the formation of gaps and clearings. We
construct spectral energy distributions (SEDs) using our IRS data as well as
published photometry and classify our sample into SED classes. Using
"extinction-free" spectral indices, we determine whether the disk, envelope, or
photosphere dominates the spectrum. We analyze the dereddened spectra of
objects which show disk dominated emission using spectral indices and
properties of silicate features in order to study the vertical and radial
structure of protoplanetary disks in NGC 1333. At least nine objects in our
sample of NGC 1333 show signs of large (several AU) radial gaps or clearings in
their inner disk. Disks with radial gaps in NGC 1333 show more-nearly pristine
silicate dust than their radially continuous counterparts. We compare
properties of disks in NGC 1333 to those in three other well studied regions,
Taurus-Auriga, Ophiuchus and Chamaeleon I, and find no difference in their
degree of sedimentation and dust processing.Comment: 67 pages, 20 figures, accepted to The Astrophysical Journal
Supplement Serie
Disk Evolution in the three Nearby Star-Forming Regions of Taurus, Chamaeleon, and Ophiuchus
We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri
stars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose
median ages lie in the <1 to 2 Myr range. The median mid-infrared spectra of
objects in these three regions are similar in shape, suggesting, on average,
similar disk structures. When normalized to the same stellar luminosity, the
medians follow each other closely, implying comparable mid-infrared excess
emission from the circumstellar disks. We use the spectral index between 13 and
31 micron and the equivalent width of the 10 micron silicate emission feature
to identify objects whose disk configuration departs from that of a continuous,
optically thick accretion disk. Transitional disks, whose steep 13-31 micron
spectral slope and near-IR flux deficit reveal inner disk clearing, occur with
about the same frequency of a few percent in all three regions. Objects with
unusually large 10 micron equivalent widths are more common (20-30%); they
could reveal the presence of disk gaps filled with optically thin dust. Based
on their medians and fraction of evolved disks, T Tauri stars in Taurus and
Chamaeleon I are very alike. Disk evolution sets in early, since already the
youngest region, the Ophiuchus core (L1688), has more settled disks with larger
grains. Our results indicate that protoplanetary disks show clear signs of dust
evolution at an age of a few Myr, even as early as ~1 Myr, but age is not the
only factor determining the degree of evolution during the first few million
years of a disk's lifetime.Comment: 23 pages, 20 figures; accepted for publication in Ap
- …