1,596 research outputs found
A spectroscopic examination of accretion diagnostics for near solar mass stars in IC 348
High-resolution optical and moderate-resolution near-infrared spectra were obtained for 40 near solar mass (~2.0-0.5 M⊙) members of the 2-3 Myr old cluster IC 348 in order to examine established accretion diagnostics and the coupling between inner disk gas and hot, micron-sized dust grains inferred from thermal and mid-infrared excesses. The stellar sample was drawn from the cluster census of Luhman in 2003 with membership being confirmed by radial velocity analysis and the presence of strong Li I λ6708 absorption. Of the stars included in this survey, 12 were classified by Lada in 2006 as hosting primordial, optically thick circumstellar disks, 5 as weak or transition disk systems, and 23 as non-excess stars using the measured slope of the stellar spectral energy distribution (SED) through the four Infrared Array Camera channels (3.6-8.0 μm) of Spitzer Space Telescope. Using the velocity width of Hα as an accretion indicator, we find that 11 primordial disk candidates are suspected accretors, suggesting a strong correlation between gaseous inner disks and optically thick dust emission. Of the five weak or transition disk systems observed, two (L21 and L67) exhibit spectroscopic features indicative of accretion. The presence of gas within the inner disk of these systems, which are free of infrared excess emission shortward of ~4.5 μm, may place constraints upon the physical mechanism responsible for inner disk clearing. Mass accretion rates (M) were determined for all suspected accretors using continuum excess measurements near λ6500 and established relationships between He I λ5876, Hα, Ca II λ8542, Paβ, and Brγ line fluxes and accretion luminosity. M values were found to range from log M to –7.2 M⊙ yr^–1, with a median value of –8.1 M⊙ yr^–1. Magnetospheric accretion models of Hα, Paβ, and Brγ emission by Muzerolle et al. and Kurosawa et al. are found to be in relative agreement with observed fluxes and derived M estimates. He I λ10830 with its metastable lower level is confirmed to be a critical indicator of magnetospheric accretion, although deep subcontinuum absorption profiles often associated with its emission are not fully understood. No statistically significant correlation is found between M and the slope of the SED from 3.6 to 8.0 μm or from 8.0 to 24.0 μm; however, the small number of suspected accretors examined does not allow broader conclusions to be drawn
Spitzer spectroscopy of circumstellar disks in the 5 Myr old upper Scorpius OB association
We present mid-infrared spectra between 5.2 and 38 μm for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early-type (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 μm. In contrast, 10 and 20 μm silicate features or polycyclic aromatic hydrocarbon emission are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral-type dependent, with the most prominent features being associated with K5-M2-type stars. By fitting the spectral energy distributions (SED) of a representative sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high-resolution (R ~ 33,000) optical (λλ4800-9200) spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only seven (all late-type) exhibit definitive signatures of accretion. Mass-accretion rates (M) for these stars were estimated to range from 10^–11 to 10^–8.9 M⊙ yr^–1. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near- and mid-infrared excess emission and an order of magnitude lower mass-accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time
An Optical Survey of the Partially Embedded Young Cluster in NGC 7129
NGC 7129 is a bright reflection nebula located in the molecular cloud complex
near l=105.4, b=+9.9, about 1.15 kpc distant. Embedded within the reflection
nebula is a young cluster dominated by a compact grouping of four early-type
stars: BD+65 1638 (B3V), BD+65 1637 (B3e), SVS 13 (B5e), and LkH-alpha 234
(B8e). About 80 H-alpha emission sources brighter than V~23 are identified in
the region, many of which are presumably T Tauri star members of the cluster.
We also present deep (V~23), optical (VRI) photometry of a field centered on
the reflection nebula and spectral types for more than 130 sources determined
from low dispersion, optical spectroscopy. The narrow pre-main sequence evident
in the color-magnitude diagram suggests that star formation was rapid and
coeval. A median age of about 1.8 Myr is inferred for the H-alpha and
literature-identified X-ray emission sources having established spectral types,
using pre-main sequence evolutionary models. Our interpretation of the
structure of the molecular cloud and the distribution of young stellar objects
is that BD+65 1638 is primarily responsible for evacuating the blister-like
cavity within the molecular cloud. LkH-alpha 234 and several embedded sources
evident in near infrared adaptive optics imaging have formed recently within
the ridge of compressed molecular gas. The compact cluster of low-mass stars
formed concurrently with the early-type members, concentrated within a central
radius of ~0.7 pc. Star formation is simultaneously occurring in a
semi-circular arc some ~3 pc in radius that outlines remaining dense regions of
molecular gas. High dispersion, optical spectra are presented for BD+65 1638,
BD+65 1637, SVS 13, LkH-alpha 234, and V350 Cep. These spectra are discussed in
the context of the circumstellar environments inferred for these stars.Comment: 45 pages, 20 figures, accepted for publication in the Astronomical
Journa
Induced Kramer-Pesch-Effect in a Two Gap Superconductor: Application to MgB2
The size of the vortex core in a clean superconductor is strongly temperature
dependent and shrinks with decreasing temperature, decreasing to zero for T ->
0. We study this so-called Kramer-Pesch effect both for a single gap
superconductor and for the case of a two gap superconductor using parameters
appropriate for Magnesium Diboride. Usually, the Kramer-Pesch effect is absent
in the dirty limit. Here, we show that the Kramer-Pesch effect exists in both
bands of a two gap superconductor even if only one of the two bands is in the
clean limit and the other band in the dirty limit, a case appropriate for MgB2.
In this case an induced Kramer-Pesch effect appears in the dirty band. Besides
numerical results we also present an analytical model for the spatial variation
of the pairing potential in the vicinity of the vortex center that allows a
simple calculation of the vortex core radius even in the limit T -> 0.Comment: 12 pages, 12 figure
High temperature superconductivity in dimer array systems
Superconductivity in the Hubbard model is studied on a series of lattices in
which dimers are coupled in various types of arrays. Using fluctuation exchange
method and solving the linearized Eliashberg equation, the transition
temperature of these systems is estimated to be much higher than that of
the Hubbard model on a simple square lattice, which is a model for the high
cuprates. We conclude that these `dimer array' systems can generally
exhibit superconductivity with very high . Not only -electron systems,
but also -electron systems may provide various stages for realizing the
present mechanism.Comment: 4 pages, 9 figure
Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr
We present results from a mid-infrared imaging survey of the ~5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant Hα emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from ~10 to 0.3 M_⊙ (B2-M5 spectral types) was drawn from known Hα emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of ~7% ± 2%, with another ~12% ± 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than ~1.2 M_⊙ exhibit significant infrared excess shortward of 8 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 4% for stars with masses between 1.05 and 0.6 M_⊙, but the Spitzer IRAC survey is sensitivity-limited below ~0.3 M_⊙. From Hα emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at ~5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKM-type stars
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