554 research outputs found
Structural variations in metal complexes of a tertiary α-hydroxyoxime
Despite the long term interest in hydroxyoximes as metal ion extractants, there is a lack of information on the possible coordination modes these ligands can assume, particularly in concert with a co-ligand. This is pertinent to the use of these extractants in synergistic systems, where a combination of extractants can achieve commercially useful results. We report here the structures of some metal complexes (M = Mn, Co, Ni, Cu, and Zn) with (1-hydroxycyclohexyl)-phenyl ketone oxime. The results demonstrate that this ligand can support complexes ranging from mononuclear to trinuclear, in association with anionic and neutral co-ligands in some cases. While these results have been obtained in the solid state, they illustrate a range of possible species that may be formed in extractant solutions
A Spitzer view of protoplanetary disks in the gamma Velorum cluster
We present new Spitzer Space Telescope observations of stars in the young ~5
Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we
have selected 579 stars as candidate members of the cluster. With the addition
of the Spitzer mid-infrared data, we have identified 5 debris disks around
A-type stars, and 5-6 debris disks around solar-type stars, indicating that the
strong radiation field in the cluster does not completely suppress the
production of planetesimals in the disks of cluster members. However, we find
some evidence that the frequency of circumstellar primordial disks is lower,
and the IR flux excesses are smaller than for disks around stellar populations
with similar ages. This could be evidence for a relatively fast dissipation of
circumstellar dust by the strong radiation field from the highest mass star(s)
in the cluster. Another possibility is that gamma Velorum stellar cluster is
slightly older than reported ages and the the low frequency of primordial disks
reflects the fast disk dissipation observed at ~5 Myr.Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11
Figure
Unveiling the Structure of Pre-Transitional Disks
In the past few years, several disks with inner holes that are empty of small
dust grains have been detected and are known as transitional disks. Recently,
Spitzer has identified a new class of "pre-transitional disks" with gaps; these
objects have an optically thick inner disk separated from an optically thick
outer disk by an optically thin disk gap. A near-infrared spectrum provided the
first confirmation of a gap in the pre-transitional disk of LkCa 15 by
verifying that the near-infrared excess emission in this object was due to an
optically thick inner disk. Here we investigate the difference between the
nature of the inner regions of transitional and pre-transitional disks using
the same veiling-based technique to extract the near-infrared excess emission
above the stellar photosphere. We show that the near-infrared excess emission
of the previously identified pre-transitional disks of LkCa 15 and UX Tau A in
Taurus as well as the newly identified pre-transitional disk of ROX 44 in
Ophiuchus can be fit with an inner disk wall located at the dust destruction
radius. We also model the broad-band SEDs of these objects, taking into account
the effect of shadowing by the inner disk on the outer disk, considering the
finite size of the star. The near-infrared excess continua of these three
pre-transitional disks, which can be explained by optically thick inner disks,
are significantly different from that of the transitional disks of GM Aur,
whose near-infrared excess continuum can be reproduced by emission from
sub-micron-sized optically thin dust, and DM Tau, whose near-infrared spectrum
is consistent with a disk hole that is relatively free of small dust. The
structure of pre-transitional disks may be a sign of young planets forming in
these disks and future studies of pre-transitional disks will provide
constraints to aid in theoretical modeling of planet formation.Comment: Accepted for publication in ApJ on May 10, 2010; 29 page
Emission from Water Vapor and Absorption from Other Gases at 5-7.5 Microns in Spitzer-IRS Spectra of Protoplanetary Disks
We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming
region showing emission in Spitzer Space Telescope Infrared Spectrograph (IRS)
5-7.5 micron spectra from water vapor and absorption from other gases in these
stars' protoplanetary disks. Seven stars' spectra show an emission feature at
6.6 microns due to the nu_2 = 1-0 bending mode of water vapor, with the shape
of the spectrum suggesting water vapor temperatures > 500 K, though some of
these spectra also show indications of an absorption band, likely from another
molecule. This water vapor emission contrasts with the absorption from warm
water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other
six of the thirteen stars have spectra showing a strong absorption band,
peaking in strength at 5.6-5.7 microns, which for some is consistent with
gaseous formaldehyde (H2CO) and for others is consistent with gaseous formic
acid (HCOOH). There are indications that some of these six stars may also have
weak water vapor emission. Modeling of these stars' spectra suggests these
gases are present in the inner few AU of their host disks, consistent with
recent studies of infrared spectra showing gas in protoplanetary disks.Comment: 33 pages, 9 figures, to appear in the 20 August, 2014, V791 - 2 issue
of the Astrophysical Journa
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The Spitzer Infrared Spectrograph Survey of T Tauri Stars in Taurus
We present 161 Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars and young brown dwarfs in the Taurus star-forming region. All of the targets were selected based on their infrared excess and are therefore surrounded by protoplanetary disks; they form the complete sample of all available IRS spectra of T Tauri stars with infrared excesses in Taurus. We also present the IRS spectra of seven Class 0/I objects in Taurus to complete the sample of available IRS spectra of protostars in Taurus. We use spectral indices that are not significantly affected by extinction to distinguish between envelope-and disk-dominated objects. Together with data from the literature, we construct spectral energy distributions for all objects in our sample. With spectral indices derived from the IRS spectra we infer disk properties such as dust settling and the presence of inner disk holes and gaps. We find a transitional disk frequency, which is based on objects with unusually large 13-31 mu m spectral indices indicative of a wall surrounding an inner disk hole, of about 3%, and a frequency of about 20% for objects with unusually large 10 mu m features, which could indicate disk gaps. The shape and strength of the 10 mu m silicate emission feature suggests weaker 10 mu m emission and more processed dust for very low mass objects and brown dwarfs (spectral types M6-M9). These objects also display weaker infrared excess emission from their disks, but do not appear to have more settled disks than their higher-mass counterparts. We find no difference for the spectral indices and properties of the dust between single and multiple systems.NASANASA through JPL/CaltechNASA through the Spitzer Space TelescopeNational Science Foundation AST-0544588, 0901947Pennsylvania State UniversityEberly College of SciencePennsylvania Space Grant ConsortiumNSFAstronom
A slowly accreting ~10 Myr old transitional disk in Orion OB1a
Here we present the Spitzer IRS spectrum of CVSO 224, the sole transitional
disk located within the ~10 Myr old 25 Orionis group in Orion OB1a. A model fit
to the spectral energy distribution of this object indicates a ~7 AU inner disk
hole that contains a small amount of optically thin dust. In previous studies,
CVSO 224 had been classified as a weak-line T Tauri star based on its Halpha
equivalent width, but here we find an accretion rate of 7x10^-11 Msun/yr based
on high-resolution Hectochelle data. CVSO 224's low mass accretion rate is in
line with photoevaporative clearing theories. However, the Spitzer IRS spectrum
of CVSO 224 has a substantial mid-infrared excess beyond 20microns which
indicates that it is surrounded by a massive outer disk. Millimeter
measurements are necessary to constrain the mass of the outer disk around CVSO
224 in order to confirm that photoevaporation is not the mechanism behind
creating its inner disk hole.Comment: 4 pages, accepted to ApJ
The Structure of the DoAr 25 Circumstellar Disk
We present high spatial resolution (< 0.3" = 40\Sigma
\propto r^{-p}$ with p = 0.34, significantly less steep than a steady-state
accretion disk (p = 1) or the often adopted minimum mass solar nebula (p =
1.5). Even though the total mass of material is large (M_d = 0.10 M_sun), the
densities inferred in the inner disk for such a model may be too low to
facilitate any mode of planet formation. However, alternative models with
steeper density gradients (p = 1) can explain the observations equally well if
substantial grain growth in the planet formation region (r < 40 AU) has
occurred. We discuss these data in the context of such models with dust
properties that vary with radius and highlight their implications for
understanding disk evolution and the early stages of planet formation.Comment: ApJL in pres
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