75 research outputs found
High Spatial Resolution Observations of Two Young Protostars in the R Corona Australis Region
We present multi-wavelength, high spatial resolution imaging of the IRS 7
region in the R Corona Australis molecular cloud. Our observations include 1.1
mm continuum and HCO^+ J = images from the SMA, ^{12}CO J =
outflow maps from the DesertStar heterodyne array receiver on the HHT, 450
m and 850 m continuum images from SCUBA, and archival Spitzer IRAC
and MIPS 24 \micron images. The accurate astrometry of the IRAC images allow us
to identify IRS 7 with the cm source VLA 10W (IRS 7A) and the X-ray source X_W.
The SMA 1.1 mm image reveals two compact continuum sources which are also
distinguishable at 450 m. SMA 1 coincides with X-ray source CXOU
J190156.4-365728 and VLA cm source 10E (IRS 7B) and is seen in the IRAC and
MIPS images. SMA 2 has no infrared counterpart but coincides with cm source VLA
9. Spectral energy distributions constructed from SMA, SCUBA and Spitzer data
yield bolometric temperatures of 83 K for SMA 1 and 70 K for SMA 2. These
temperatures along with the submillimeter to total luminosity ratios indicate
that SMA 2 is a Class 0 protostar, while SMA 1 is a Class 0/Class I
transitional object (L= \Lsun). The ^{12}CO J = outflow map
shows one major and possibly several smaller outflows centered on the IRS 7
region, with masses and energetics consistent with previous work. We identify
the Class 0 source SMA 2/VLA 9 as the main driver of this outflow. The complex
and clumpy spatial and velocity distribution of the HCO^+ J =
emission is not consistent with either bulk rotation, or any known molecular
outflow activity.Comment: 31 pages, 8 figures, Accepted to Ap
GQ Lup B Visible & Near-Infrared Photometric Analysis
We have re-analyzed archival HST and Subaru data of the recently discovered
planetary mass companion (PMC) GQ Lup B. With these we produce the first R and
I band photometry of the companion and fit a radius and effective temperature
using detailed model atmospheres. We find an effective temperature of 2335 +/-
100K, and a radius of 0.38 +/- 0.05 Rsol and luminosity of log(L/Lsol) = -2.42
+/- 0.07 (at 140pc). Since we fit wavelengths that span most of the emitted
radiation from GQ Lup this luminosity estimate is robust, with uncertainty
dominated by the distance uncertainty (+/- 50 pc). The radius obtained for
140pc (0.38Rsol) is significantly larger than the one originally derived and
larger than model predictions. The mass of the object is much more
model-dependent than the radiative properties, but for the GAIA dusty models we
find a mass between 10-20 MJup, in the range of the brown dwarf and PMC
deuterium burning boundary. Assuming a distance of 140pc, observations fit to
1sigma the Baraffe evolution model for a ~15 MJup brown dwarf. Additionally,
the F606W photometric band is significantly overluminous compared to model
predictions and other brown dwarfs. Such overluminosity could be explained by a
bright Halpha emission from chromospheric activity, interaction with another
undetected companion, or accretion. Assuming that GQ Lup B has a bright Halpha
emission line, its Halpha emission strength is 10^(-1.71 +/- 0.10) Lbol,
significantly larger than field late-type dwarfs. GQ Lup B might be strongly
accreting and still be in its formation phase.Comment: 14 pages, 3 figures, ApJL final accepted versio
Quasi-periodic X-ray Flares from the Protostar YLW15
With ASCA, we have detected three X-ray flares from the Class I protostar
YLW15. The flares occurred every ~20 hours and showed an exponential decay with
time constant 30-60 ks. The X-ray spectra are explained by a thin thermal
plasma emission. The plasma temperature shows a fast-rise and slow-decay for
each flare with kT_{peak}~4-6 keV. The emission measure of the plasma shows
this time profile only for the first flare, and remains almost constant during
the second and third flares at the level of the tail of the first flare. The
peak flare luminosities L_{X,peak} were ~5-20 * 10^{31} erg s^{-1}, which are
among the brightest X-ray luminosities observed to date for Class I protostars.
The total energy released in each flare was 3-6*10^{36} ergs. The first flare
is well reproduced by the quasi-static cooling model, which is based on solar
flares, and it suggests that the plasma cools mainly radiatively, confined by a
semi-circular magnetic loop of length ~14 Ro with diameter-to-length ratio
\~0.07. The two subsequent flares were consistent with the reheating of the
same magnetic structure as of the first flare. The large-scale magnetic
structure and the periodicity of the flares imply that the reheating events of
the same magnetic loop originate in an interaction between the star and the
disk due to the differential rotation.Comment: Accepted by ApJ, 9 pages incl. 4 ps figure
MISALIGNED DISKS IN THE BINARY PROTOSTAR IRS 43
Recent high angular resolution (0.2") ALMA observations of the 1.1 mm
continuum and of HCO+ J=3-2 and HCN J=3-2 gas towards the binary protostar IRS
43 reveal multiple Keplerian disks which are significantly misaligned (
60), both in inclination and position angle and also with respect to
the binary orbital plane. Each stellar component has an associated
circumstellar disk while the binary is surrounded by a circumbinary disk.
Together with archival VLA measurements of the stellar positions over 25 years,
and assuming a circular orbit, we use our continuum measurements to determine
the binary separation, a = 74 4 AU, and its inclination, i
30. The misalignment in this system suggests that turbulence has likely
played a major role in the formation of IRS 43.Comment: 7 pages, 4 figure
Discovery of Extremely Embedded X-ray Sources in the R Coronae Australis Star Forming Core
With the XMM-Newton and Chandra observatories, we detected two extremely
embedded X-ray sources in the R Corona Australis (R CrA) star forming core,
near IRS 7. These sources, designated as XB and XA, have X-ray absorption
columns of ~3e23 cm-2 equivalent to AV ~180 mag. They are associated with the
VLA centimeter radio sources 10E and 10W, respectively. XA is the counterpart
of the near-infrared source IRS 7, whereas XB has no K-band counterpart above
19.4 mag. This indicates that XB is younger than typical Class I protostars,
probably a Class 0 protostar or in an intermediate phase between Class 0 and
Class I. The X-ray luminosity of XB varied between 29<log LX <31.2 ergs s-1 on
timescales of 3-30 months. XB also showed a monotonic increase in X-ray
brightness by a factor of two in 30 ksec during an XMM-Newton observation. The
XMM-Newton spectra indicate emission from a hot plasma with kT ~3-4 keV and
also show fluorescent emission from cold iron. Though the X-ray spectrum from
XB is similar to flare spectra from Class I protostars in luminosity and
temperature, the light curve does not resemble the lightcurves of magnetically
generated X-ray flares because the variability timescale of XB is too long and
because variations in X-ray count rate were not accompanied by variations in
spectral hardness. The short-term variation of XB may be caused by the partial
blocking of the X-ray plasma, while the month-long flux enhancement may be
driven by mass accretion.Comment: 26 pages, 8 figures, To be published in ApJ in April 200
On the Evolutionary State of the Components of the YLW 15 Binary System
We report centimeter continuum observations with the VLA and the VLBA as well
as mid-infrared observations with COMICS/SUBARU toward the components of the
YLW 15 very young binary system, VLA 1 and VLA 2. The centimeter emission of
the two components traces partially thick free-free emission, likely due to
collimated, ionized winds. VLA 1 is an embedded protostar, undetected in the
near-IR, and possibly in the Class 0 to Class I transition and powering a
Herbig-Haro outflow. Its mid-IR emission appears slightly resolved with a
diameter of ~16 AU, possibly tracing circumstellar material from both the
envelope and the disk. VLA 2 is a typical Class I object, unresolved in the
mid-IR, and is the responsible of the strong X-ray emission associated with YLW
15. The expected centimeter ''peri-stellar'' emission associated with the X-ray
emission is not detected with the VLBA at 6 cm likely due to the high optical
depth of the free-free emission. Strikingly, the near to mid-IR properties of
YLW 15 suggest that VLA 1 is a more embedded YSO, or alternatively, less
luminous than VLA 2, whereas orbital proper motions of this binary system
indicate that VLA 1 is more massive than VLA 2. This result is apparently
against the expected evolutionary scenario, where one expects that the more
massive YSO in a binary system is the more evolved and luminous YSO. Finally,
the nearby source YLW 16A is detected with the VLA, its position coincides with
reported near-IR and X-ray sources.Comment: 30 pages, 6 figures. Accepted in Astronomical Journa
Detection of 6 K gas in Ophiuchus D
Cold cores in interstellar molecular clouds represent the very first phase in
star formation. The physical conditions of these objects are studied in order
to understand how molecular clouds evolve and how stellar masses are
determined. The purpose of this study is to probe conditions in the dense,
starless clump Ophichus D (Oph D). The ground-state (1(10)-1(11)) rotational
transition of ortho-H2D+ was observed with APEX towards the density peak of Oph
D. The width of the H2D+ line indicates that the kinetic temperature in the
core is about 6 K. So far, this is the most direct evidence of such cold gas in
molecular clouds. The observed H2D+ spectrum can be reproduced with a
hydrostatic model with the temperature increasing from about 6 K in the centre
to almost 10 K at the surface. The model is unstable against any increase in
the external pressure, and the core is likely to form a low-mass star. The
results suggest that an equilibrium configuration is a feasible intermediate
stage of star formation even if the larger scale structure of the cloud is
thought to be determined by turbulent fragmentation. In comparison with the
isothermal case, the inward decrease in the temperature makes smaller, i.e.
less massive, cores susceptible to externally triggered collapse.Comment: 7 pages, 5 figures, accepted for Astronomy and Astrophysic
Centimeter Imaging of the R Coronae Australis Region
The R CrA region was observed in the 3.5 and 6.2 cm continuum with high
angular resolutions (0.6--1.7 arcseconds) using the Very Large Array. Archival
data sets were also analyzed for comparison, which provided angular resolutions
up to 0.3 arcseconds. A cluster of young stellar objects was detected, and a
rich array of star forming activities was revealed. IRS 7A showed an enhanced
outflow activity recently. The main peak of IRS 7A positionally coincides with
an X-ray source, which suggests that the X-ray emission is directly related to
the central protostar. The Class 0 source SMA 2 is associated with a double
radio source, B 9a and 9b, and seems to be driving two outflows. The B 9
complex is probably a multiple-protostar system. Both B 9a and 9b are
nonthermal radio sources with negative spectral indices. IRS 7B is a compact
radio source surrounded by an extended structure. The compact source
corresponds to the Class 0/I source SMA 1, and it is also closely associated
with an X-ray source, suggesting that magnetic activities start early in the
protostellar stage of evolution. The extended structure of IRS 7B may be a
bipolar outflow. IRS 5 was resolved into two sources with a separation of 0.9
arcseconds. Both IRS 5a and 5b display radio flares and X-ray emission,
suggesting that energetic magnetic processes are active in both members. The
month-scale active phase of IRS 5b implies that the flare activity must involve
large-scale magnetic fields. During the strong flare event of IRS 5b in 1998,
IRS 5a also showed an enhanced level of radio emission. This concurrent
activity suggests that IRS 5 may be an interacting young binary system, but the
interaction mechanism is unknown. Alternatively, what was seen in the radio
images could be a circumbinary halo.Comment: To appear in the Astrophysical Journa
Very Low-Mass Objects in the Coronet Cluster: The Realm of the Transition Disks
We present optical and IR spectra of a set of low-mass stars and brown dwarfs
in the Coronet cluster (aged ~1Myr), obtained with the multifiber spectrograph
FLAMES/VLT and IRS/Spitzer. The optical spectra reveal spectral types between
M1 and M7.5, confirm the youth of the objects (via Li 6708 A absorption), and
show the presence of accretion (via Halpha) and shocks (via forbidden line
emission). The IRS spectra, together with IR photometry from the IRAC/MIPS
instruments on Spitzer and 2MASS, confirm the presence of IR excesses
characteristic of disks around ~70% of the objects. Half of the disks do not
exhibit any silicate emission, or present flat features characteristic of large
grains. The rest of the disks show silicate emission typical of amorphous and
crystalline silicate grains a few microns in size. About 50% of the objects
with disks do not show near-IR excess emission, having "transitional" disks,
according to their classical definition. This is a very high fraction for such
a young cluster. The large number of "transitional" disks suggests lifetimes
comparable to the lifetimes of typical optically thick disks. Therefore, these
disks may not be in a short-lived phase, intermediate between Class II and
Class III objects. The median spectral energy distribution of the disks in the
Coronet cluster is also closer to a flat disk than observed for the disks
around solar-type stars in regions with similar age. The differences in the
disk morphology and evolution in the Coronet cluster could be related to fact
that these objects have very late spectral types compared to the solar-type
stars in other cluster studies. Finally, the optical spectroscopy reveals that
one of the X-ray sources is produced by a Herbig Haro object in the cloud.Comment: 51 pages, 13 figures, 10 table
The Initial Conditions of Clustered Star Formation. II. N2H+ Observations of the Ophiuchus B Core
We present a Nobeyama 45 m Radio Telescope map and Australia Telescope
Compact Array pointed observations of N2H+ 1-0 emission towards the clustered,
low mass star forming Oph B Core within the Ophiuchus molecular cloud. We
compare these data with previously published results of high resolution NH3
(1,1) and (2,2) observations in Oph B. We use 3D Clumpfind to identify emission
features in the single-dish N2H+ map, and find that the N2H+ `clumps' match
well similar features previously identified in NH3 (1,1) emission, but are
frequently offset to clumps identified at similar resolution in 850 micron
continuum emission. Wide line widths in the Oph B2 sub-Core indicate
non-thermal motions dominate the Core kinematics, and remain transonic at
densities n ~ 3 x 10^5 cm^-3 with large scatter and no trend with N(H2).
Non-thermal motions in Oph B1 and B3 are subsonic with little variation, but
also show no trend with H2 column density. Over all Oph B, non-thermal N2H+
line widths are substantially narrower than those traced by NH3, making it
unlikely NH3 and N2H+ trace the same material, but the v_LSR of both species
agree well. We find evidence for accretion in Oph B1 from the surrounding
ambient gas. The NH3/N2H+ abundance ratio is larger towards starless Oph B1
than towards protostellar Oph B2, similar to recent observational results in
other star-forming regions. Small-scale structure is found in the ATCA N2H+ 1-0
emission, where emission peaks are again offset from continuum emission. In
particular, the ~1 M_Sun B2-MM8 clump is associated with a N2H+ emission
minimum and surrounded by a broken ring-like N2H+ emission structure,
suggestive of N2H+ depletion. We find a strong general trend of decreasing N2H+
abundance with increasing N(H2) in Oph B which matches that found for NH3.Comment: 55 pages (manuscript), 15 figures, ApJ accepte
- âŠ