84 research outputs found
The First Detailed Look at a Brown Dwarf Disk
The combination of mid-infrared and recent submm/mm measurements allows us to
set up the first comprehensive spectral energy distribution (SED) of the
circumstellar material around a young Brown Dwarf. Simple arguments suggest
that the dust is distributed in the form of a disk. We compare basic models to
explore the disk parameters. The modeling shows that a flat disk geometry fits
well the observations. A flared disk explains the SED only if it has a
puffed-up inner rim and an inner gap much larger than the dust sublimation
radius. Similarities and differences with disks around T Tauri stars are
discussed.Comment: 11 pages, 1 figur
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
Wide-Field Infrared Imaging Polarimetry of the NGC 6334 Region: A Nest of Infrared Reflection Nebulae
We report the detection of eighteen infrared reflection nebulae (IRNe) in the
, , & linear polarimetric observations of the NGC 6334 massive
star-formation complex, of which 16 IRNe are new discoveries. Our images cover
180 square arcminutes, one of the widest near-infrared polarization data
in star-formation regions so far. These IRNe are most likely associated with
embedded young OB stars at different evolutionary phases, showing a variety of
sizes, morphologies, and polarization properties, which can be divided into
four categories. We argue the different nebula characteristics to be a possible
evolutionary sequence of circumstellar structures around young massive stars.Comment: 4 pages, 1 figur
Investigation of Diffuse Hard X-ray Emission from the Massive Star-Forming Region NGC 6334
Chandra ACIS-I data of the molecular cloud and HII region complex NGC 6334
were analyzed. The hard X-ray clumps detected with ASCA (Sekimoto et al. 2000)
were resolved into 792 point sources. After removing the point sources, an
extended X-ray emission component was detected over a 5x9 pc2 region, with the
0.5-8 keV absorption-corrected luminosity of 2x10^33 erg/s. The contribution
from faint point sources to this extended emission was estimated as at most ~20
%, suggesting that most of the emission is diffuse in nature. The X-ray
spectrum of the diffuse emission was observed to vary from place to place. In
tenuous molecular cloud regions with hydrogen column density of 0.5~1x10^22
cm-2, the spectrum can be represented by a thermal plasma model with
temperatures of several keV. The spectrum in dense cloud cores exhibits harder
continuum, together with higher absorption more than ~3x10^22 cm-2. In some of
such highly obscured regions, the spectrum show extremely hard continua
equivalent to a photon index of ~1, and favor non-thermal interpretation. These
results are discussed in the context of thermal and non-thermal emissions, both
powered by fast stellar winds from embedded young early-type stars through
shock transitions.Comment: 43 pages, 14 figures, accepted for publication in ApJ, A full
resolution ot the paper can be found at
http://www.astro.isas.jaxa.jp/~ezoe/ngc6334/yezoe2005apj_chandra_ngc6334.pd
The very low-mass population of the Corona Australis and Chamaeleon II star forming regions
We present the results of a deep optical survey in the Corona Australis and
Chamaeleon II star forming regions. Our optical photometry is combined with
available near- and mid-infrared photometry to identify very low-mass candidate
members in these dark clouds. In our Chamaeleon II field, only one object
exhibits clear H-alpha emission, but the discrepancy between its optical and
near-infrared colours suggests that it might be a foreground star. We also
identify two objects without H-alpha emission that could be planetary mass
members of Chamaeleon II. In Corona Australis, we find ten stars and three
brown dwarf candidates in the Coronet cluster. Five of our new members are
identified with ISOCAM sources. Only two of them have a mid-infrared excess,
indicating the presence of an accretion disk. On the other hand, one brown
dwarf candidate has a faint close companion, seen only in our deepest I-band
image. For many of the candidates in both clouds, membership could not be
inferred from their H-alpha emission or near-infrared colours; these objects
need spectroscopic confirmation of their status
Photometric variability of young brown dwarfs in the sigma Orionis open cluster
We have carried out multi-epoch, time-series differential I-band photometry
of a large sample of objects in the south-east region of the young (~3 Myr),
nearby (~350 pc) sigma Orionis open cluster. A field of ~1000 arcmin^2 was
monitored during four nights over a period of two years. Using this dataset, we
have studied the photometric variability of twenty-eight brown dwarf cluster
candidates with masses ranging from the stellar-substellar boundary down to the
planetary-mass domain. We have found that about 50% of the sample show
photometric variability on timescales from less than one hour to several days
and years. The amplitudes of the I-band light curves range from less than 0.01
up to ~0.4 magnitudes. A correlation between the near-infrared excess in the
K_s band, strong Halpha emission and large-amplitude photometric variation is
observed. We briefly discuss how these results may fit the different scenarios
proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic
spots, patchy obscuration by dust clouds, surrounding accretion discs and
binarity). Additionally, we have determined tentative rotational periods in the
range 3 to 40 h for three objects with masses around 60 M_Jup, and the
rotational velocity of 14+/-4 km/s for one of them.Comment: Accepted for publication in A&
The brown dwarf population in the Chamaeleon I cloud
We present the results of a multiband survey for brown dwarfs in the
Chamaeleon I dark cloud with the Wide Field Imager (WFI) camera at the ESO/MPG
2.2-m telescope on La Silla (Chile). The survey has revealed a substantial
population of brown dwarfs in this southern star forming region. Candidates
were selected from R, I and H-alpha imaging observations. We also observed in
two medium-band filters, M855 and M915, for the purpose of spectral type
determination. The former filter covers a wavelength range containing spectral
features characteristic of M-dwarfs, while the latter lies in a relatively
featureless wavelength region for these late-type objects. A correlation was
found between spectral type and (M855-M915) colour index for mid- to late
M-type objects and early L-type dwarfs. With this method, we identify most of
our object candidates as being of spectral type M5 or later. Our results show
that there is no strong drop in the number of objects for the latest spectral
types, hence brown dwarfs may be as abundant as low-mass stars in this region.
Also, both kind of objects have a similar spatial distribution. We derive an
index of the mass function in this region of dispersed
star formation, in good agreement with the values obtained in other star
forming regions and young clusters. Some of the brown dwarfs have strong
H-alpha emission, suggesting mass accretion. For objects with published
infrared photometry, we find that strong H-alpha emission is related to a
mid-infrared excess, indicative of the existence of a circumstellar disk.Comment: Accepted for publication in Astronomy & Astrophysic
Re-evaluation of the Fijianolide/Laulimalide Chemotype Suggests an Alternate Mechanism of Action for C-15/C-20 Analogs.
Herein, we report on naturally derived microtubule stabilizers with activity against triple negative breast cancer (TNBC) cell lines, including paclitaxel, fijianolide B/laulimalide (3), fijianolide B di-acetate (4), and two new semisynthetic analogs of 3, which include fijianolide J (5) and fijianolide L (6). Similar to paclitaxel, compound 3 demonstrated classic microtubule stabilizing activity with potent (GI50 = 0.7–17 nM) antiproliferative efficacy among the five molecularly distinct TNBC cell lines. Alternatively, compounds 5 or 6, generated from oxidation of C-20 or C-15 and C-20 respectively, resulted in a unique profile with reduced potency (GI50 = 4–9 μM), but improved efficacy in some lines, suggesting a distinct mechanism of action. The C-15, C-20 di-acetate, and dioxo modifications on 4 and 6 resulted in compounds devoid of classic microtubule stabilizing activity in biochemical assays. While 4 also had no detectable effect on cellular microtubules, 6 promoted a reorganization of the cytoskeleton resulting in an accumulation of microtubules at the cell periphery. Compound 5, with a single C-20 oxo substitution, displayed a mixed phenotype, sharing properties of 3 and 6. These results demonstrate the importance of the C-15/C-20 chiral centers, which appear to be required for the potent microtubule stabilizing activity of this chemotype and that oxidation of these sites promotes unanticipated cytoskeletal alterations that are distinct from classic microtubule stabilization, likely through a distinct mechanism of action
A WFI survey in the Chamaeleon II dark cloud
We present the results of an optical multi-band survey for low-mass Pre-Main
Sequence (PMS) stars and young Brown Dwarfs (BDs) in the Chamaeleon II (Cha II)
dark cloud. This survey constitutes the complementary optical data to the c2d
Spitzer Legacy survey in Cha II.
Using the Wide-Field Imager (WFI) at the ESO 2.2m telescope, we surveyed a
sky area of about 1.75 square degrees in Cha II. The region was observed in the
Rc, Ic and z broad-bands, in H-alpha and in two medium-band filters centered at
856 and 914 nm. We select PMS star and young BD candidates using
colour-magnitude diagrams (CMDs) and theoretical isochrones reproduced ad-hoc
for the WFI at the ESO2.2m telescope system. The selection criteria are also
reinforced by using the previously known PMS stars in Cha II to define the PMS
locus on the CMDs and by investigating the infrared (IR) colours of the
candidates. By exploiting the WFI intermediate-band photometry we also estimate
the effective temperature and the level of H-alpha emission of the candidates.
Our survey, which is one of the largest and deepest optical surveys conducted
so far in Cha II, recovered the majority of the PMS stars and 10 member
candidates of the cloud from previous IR surveys. In addition, the survey
revealed 10 new potential members. From our photometric characterisation, we
estimate that some 50% of the 20 candidates will result in true Cha II members.
Based on our temperature estimates, we conclude that several of these objects
are expected to be sub-stellar and give a first estimate of the fraction of
sub-stellar objects.Comment: 26 pages, 18 figure
3-He in the Milky Way Interstellar Medium: Ionization Structure
The cosmic abundance of the 3-He isotope has important implications for many
fields of astrophysics. We are using the 8.665 GHz hyperfine transition of
3-He+ to determine the 3-He/H abundance in Milky Way HII regions and planetary
nebulae. This is one in a series of papers in which we discuss issues involved
in deriving accurate 3-He/H abundance ratios from the available measurements.
Here we describe the ionization correction we use to convert the 3-He+/H+
abundance, y3+, to the 3-He/H abundance, y3. In principle the nebular
ionization structure can significantly influence the y3 derived for individual
sources. We find that in general there is insufficient information available to
make a detailed ionization correction. Here we make a simple correction and
assess its validity. The correction is based on radio recombination line
measurements of H+ and 4-He+, together with simple core-halo source models. We
use these models to establish criteria that allow us to identify sources that
can be accurately corrected for ionization and those that cannot. We argue that
this effect cannot be very large for most of the sources in our observational
sample. For a wide range of models of nebular ionization structure we find that
the ionization correction factor varies from 1 to 1.8. Although large
corrections are possible, there would have to be a conspiracy between the
density and ionization structure for us to underestimate the ionization
correction by a substantial amount.Comment: 36 pages, 4 figures To appear Astrophysical Journal, 20 August 2007,
vol 665, no
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