886 research outputs found
The Stellar Population of the Chamaeleon I Star-Forming Region
I present a new census of the stellar population in the Chamaeleon I
star-forming region. Using optical and near-IR photometry and followup
spectroscopy, I have discovered 50 new members of Chamaeleon I, expanding the
census of known members to 226 objects. Fourteen of these new members have
spectral types later than M6, which doubles the number of known members that
are likely to be substellar. I have estimated extinctions, luminosities, and
effective temperatures for the known members, used these data to construct an
H-R diagram for the cluster, and inferred individual masses and ages with the
theoretical evolutionary models of Baraffe and Chabrier. The distribution of
isochronal ages indicates that star formation began 3-4 and 5-6 Myr ago in the
southern and northern subclusters, respectively, and has continued to the
present time at a declining rate. The IMF in Chamaeleon I reaches a maximum at
a mass of 0.1-0.15 M_sun, and thus closely resembles the IMFs in IC 348 and the
Orion Nebula Cluster. In logarithmic units where the Salpeter slope is 1.35,
the IMF is roughly flat in the substellar regime and shows no indication of
reaching a minimum down to a completeness limit of 0.01 M_sun. The low-mass
stars are more widely distributed than members at other masses in the northern
subcluster, but this is not the case in the southern subcluster. Meanwhile, the
brown dwarfs have the same spatial distribution as the stars out to a radius of
3 deg (8.5 pc) from the center of Chamaeleon I
On the stability of very massive primordial stars
The stability of metal-free very massive stars ( = 0; M = 120 - 500
\msol) is analyzed and compared with metal-enriched stars. Such zero-metal
stars are unstable to nuclear-powered radial pulsations on the main sequence,
but the growth time scale for these instabilities is much longer than for their
metal-rich counterparts. Since they stabilize quickly after evolving off the
ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss
in Z = 0 stars. For reasonable assumptions regarding the efficiency of
converting pulsational energy into mass loss, we find that, even for the larger
masses considered, the star may die without losing a large fraction of its
mass. We find a transition between the - and -mechanisms for
pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich
stars, the -mechanism yields much shorter -folding times, indicating
the presence of a strong instability. We thus stress the fundamental difference
of the stability and late stages of evolution between very massive stars born
in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1,
accepted for publication in Ap
A Census of the Chamaeleon I Star-Forming Region
Optical spectroscopy has been obtained for 179 objects that have been
previously identified as possible members of the cluster, that lack either
accurate spectral types or clear evidence of membership, and that are optically
visible (I<18). I have used these spectroscopic data and all other available
constraints to evaluate the spectral classifications and membership status of a
total sample of 288 candidate members of Chamaeleon I that have appeared in
published studies of the cluster. The latest census of Chamaeleon I now
contains 158 members, 8 of which are later than M6 and thus are likely to be
brown dwarfs. I find that many of the objects identified as members of
Chamaeleon I in recent surveys are actually field stars. Meanwhile, 7 of 9
candidates discovered by Carpenter and coworkers are confirmed as members, one
of which is the coolest known member of Chamaeleon I at a spectral type of M8
(~0.03 M_sun). I have estimated extinctions, luminosities, and effective
temperatures for the members and used these data to construct an H-R diagram
for the cluster. Chamaeleon I has a median age of ~2 Myr according to
evolutionary models, and hence is similar in age to IC 348 and is slightly
older than Taurus (~1 Myr). The measurement of an IMF for Chamaeleon I from
this census is not possible because of the disparate methods with which the
known members were originally selected, and must await an unbiased,
magnitude-limited survey of the cluster.Comment: 59 pages, 22 figure
Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I
We report the discovery of quiescent emission from molecular hydrogen gas
located in the circumstellar disks of six pre-main sequence stars, including
two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon
I star forming region. For two of these stars, we also place upper limits on
the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence
sources now known to be sources of quiescent near-infrared hydrogen emission,
four possess transitional disks, which suggests that detectable levels of H
emission and the presence of inner disk holes are correlated. These H
detections demonstrate that these inner holes are not completely devoid of gas,
in agreement with the presence of observable accretion signatures for all four
of these stars and the recent detections of [Ne II] emission from three of
them. The overlap in [Ne II] and H detections hints at a possible
correlation between these two features and suggests a shared excitation
mechanism of high energy photons. Our models, combined with the kinematic
information from the H lines, locate the bulk of the emitting gas at a few
tens of AU from the stars. We also find a correlation between H detections
and those targets which possess the largest H equivalent widths,
suggesting a link between accretion activity and quiescent H emission. We
conclude that quiescent H emission from relatively hot gas within the disks
of TTS is most likely related to on-going accretion activity, the production of
UV photons and/or X-rays, and the evolutionary status of the dust grain
populations in the inner disks.Comment: 12 pages, emulateapj, Accepted by Ap
Inactivation of mammalian Ero 1α is catalysed by specific protein disulfide isomerases
Disulfide formation within the endoplasmic reticulum is a complex process requiring a disulfide exchange protein such as protein disulfide isomerase and a mechanism to form disulfides de novo. In mammalian cells, the major pathway for de novo disulfide formation involves the enzyme Ero1α which couples oxidation of thiols to the reduction of molecular oxygen to form hydrogen peroxide. Ero1α activity is tightly regulated by a mechanism that requires the formation of regulatory disulfides. These regulatory disulfides are reduced to activate and reform to inactive the enzyme. To investigate the mechanism of inactivation we analysed regulatory disulfide formation in the presence of various oxidants under controlled oxygen concentration. Neither molecular oxygen, nor hydrogen peroxide was able to oxidise Ero1α efficiently to form the correct regulatory disulfides. However, specific members of the PDI family such as PDI or ERp46 were able to catalyse this process. Further studies showed that both active sites of PDI contribute to the formation of regulatory disulfides in Ero1α and that the PDI substrate binding domain is crucial to allow electron transfer between the two enzymes. These results demonstrate a simple feedback mechanism of regulation of mammalian Ero1α involving its primary substrate
Observation of 1D Behavior in Si Nanowires: Toward High-Performance TFETs
This article provides experimental evidence of one-dimensional behavior of silicon (Si) nanowires (NWs) at low-temperature through both transfer (IdâVG) and capaci- tanceâvoltage characteristics. For the first time, operation of Si NWs in the quantum capacitance limit (QCL) is experimentally demonstrated and quantitatively analyzed. This is of relevance since working in the QCL may allow, e.g., tunneling field-effect transistors (TFETs) to achieve higher on-state currents (Ion) and larger on-/off-state current ratios (Ion/Ioff), thus addressing one of the most severe limitations of TFETs. Comparison of the experimental data with simulations finds excellent agreement using a simple capacitor model
Statistical Properties of Galactic Starlight Polarization
We present a statistical analysis of Galactic interstellar polarization from
the largest compilation available of starlight data. The data comprises ~ 9300
stars of which we have selected ~ 5500 for our analysis. We find a nearly
linear growth of mean polarization degree with extinction. The amplitude of
this correlation shows that interstellar grains are not fully aligned with the
Galactic magnetic field, which can be interpreted as the effect of a large
random component of the field. In agreement with earlier studies of more
limited scope, we estimate the ratio of the uniform to the random
plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8.
Moreover, a clear correlation exists between polarization degree and
polarization angle what provides evidence that the magnetic field geometry
follows Galactic structures on large-scales. The angular power spectrum C_l of
the starlight polarization degree for Galactic plane data (|b| < 10 deg) is
consistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/\theta is the
multipole order), for all angular scales \theta > 10 arcmin. An investigation
of sparse and inhomogeneous sampling of the data shows that the starlight data
analyzed traces an underlying polarized continuum that has the same power
spectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can be
safely used for the modeling of Galactic polarized continuum emission at other
wavelengths.Comment: 31 pages, 11 figures. Minor corrections and some clarifications
included. Matches version accepted for publication by the Astrophysical
Journa
- âŠ