49 research outputs found
Molecules in the transition disk orbiting T Cha
We seek to establish the presence and properties of gas in the circumstellar
disk orbiting T Cha, a nearby (d~110 pc), relatively evolved (age ~5-7 Myr) yet
actively accreting 1.5 Msun T Tauri star. We used the APEX 12 m radiotelescope
to search for submillimeter molecular emission from the T Cha disk, and we
reanalyzed archival XMM-Newton spectroscopy of T Cha to ascertain the
intervening absorption due to disk gas along the line of sight to the star
(N_H). We detected submillimeter rotational transitions of 12CO, 13CO, HCN, CN
and HCO+ from the T Cha disk. The 12CO line appears to display a double-peaked
line profile indicative of Keplerian rotation. Analysis of the CO emission line
data indicates that the disk around T Cha has a mass (M_disk,H_2 = 80 M_earth)
similar to, but more compact (R_disk, CO~80 AU) than, other nearby, evolved
molecular disks (e.g. V4046 Sgr, TW Hya, MP Mus) in which cold molecular gas
has been previously detected. The HCO+/13CO and HCN/13CO, line ratios measured
for T Cha appear similar to those of other evolved circumstellar disks (i.e. TW
Hya and V4046 Sgr), while the CN/13CO ratio appears somewhat weaker. Analysis
of the XMM-Newton data shows that the atomic absorption toward T Cha is
1-2 orders of magnitude larger than toward the other nearby T Tauri with
evolved disks. Furthermore, the ratio between atomic absorption and optical
extinction N_H/A_V toward T Cha is higher than the typical value observed for
the interstellar medium and young stellar objects in the Orion Nebula Cluster.
This may suggest that the fraction of metals in the disk gas is higher than in
the interstellar medium. Our results confirm that pre-main sequence stars older
than ~5 Myr, when accreting, retain cold molecular disks, and that those
relatively evolved disks display similar physical and chemical properties.Comment: Accepted for publication on A&
Kinematic and morphological modeling of the bipolar nebula Sa2-237
We present [OIII]500.7nm and Halpha+[NII] images and long-slit, high
resolution echelle spectra in the same spectral regions of Sa2--237, a possible
bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent,
with a narrow waist, and showing strong point symmetry about the central
object, indicating it's likely binary nature. The long slit spectra were taken
over the long axis of the nebula, and show a distinct ``eight'' shaped pattern
in the velocity--space plot, and a maximum projected outflow velocity of
V=106km/s, both typical of expanding bipolar planetary nebulae. By model
fitting the shape and spectrum of the nebula simultaneously, we derive the
inclination of the long axis to be 70 degrees, and the maximum space velocity
of expansion to be 308 km/s. Due to asymmetries in the velocities we adopt a
new value for the system's heliocentric radial velocity of -30km/s. We use the
IRAS and 21cm radio fluxes, the energy distribution, and the projected size of
Sa2-237 to estimate it's distance to be 2.1+-0.37kpc. At this distance Sa2-237
has a luminosity of 340 Lsun, a size of 0.37pc, and -- assuming constant
expansion velocity -- a nebular age of 624 years. The above radial velocity and
distance place Sa2--237 in the disk of the Galaxy at z=255pc, albeit with
somewhat peculiar kinematics.Comment: 10pp, 4 fig
The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions
We present an overview of the initial results from the Chandra Planetary
Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray
Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The
first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc
of Earth, yielding 4 detections of diffuse X-ray emission and 9 detections of
X-ray-luminous point sources at the central stars (CSPNe) of these objects.
Combining these results with those obtained from Chandra archival data for all
(14) other PNe within ~1.5 kpc that have been observed to date, we find an
overall X-ray detection rate of ~70%. Roughly 50% of the PNe observed by
Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing
shocks formed by energetic wind collisions is detected in ~30%; five objects
display both diffuse and point-like emission components. The presence of X-ray
sources appears correlated with PN density structure, in that molecule-poor,
elliptical nebulae are more likely to display X-ray emission (either point-like
or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of
the X-ray point sources detected at CSPNe display X-ray spectra that are harder
than expected from hot (~100 kK) central star photospheres, possibly indicating
a high frequency of binary companions to CSPNe. Other potential explanations
include self-shocking winds or PN mass fallback. Most PNe detected as diffuse
X-ray sources are elliptical nebulae that display a nested shell/halo structure
and bright ansae; the diffuse X-ray emission regions are confined within inner,
sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have
inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the
timescale for strong shocks due to wind interactions in PNe.Comment: 41 pages, 6 figures; submitted to the Astronomical Journa
X-ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-ray Observatory (CXO) observations of the
planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type
Wolf-Rayet central stars that are presently driving fast ~1000 km/s, massive
winds into denser, slow-moving (~10 km/s) material ejected during recently
terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these
observations provide key tests of models of wind-wind interactions in PNs. In
NGC 40, we detect faint, diffuse X-ray emission distributed within a partial
annulus that lies nested within a ~40'' diameter ring of nebulosity observed in
optical and near-infrared images. Hen 2-99 is undetected. The inferred X-ray
temperature (T_X ~10^6 K) and luminosity (L_X ~ 2 X 10^30 ergs/s) of NGC 40 are
the lowest measured thus far for any PN displaying diffuse X-ray emission.
These results, combined with the ring-like morphology of the X-ray emission
from NGC 40, suggest that its X-ray emission arises from a ``hot bubble'' that
is highly evolved and is generated by a shocked, quasi-spherical fast wind from
the central star, as opposed to AGB or post-AGB jet activity. In constrast, the
lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet
to enter a phase of strong wind-wind shocks.Comment: 15 pages, 5 figures to appear in The Astrophysical Journa
Early results from ChanPLaNS: mystery of hard X-ray emitting CSPNe
We are presently using the Chandra X-ray Observatory to conduct the first systematic X-ray survey of planetary nebulae (PNe) in the solar neighborhood. The Chandra Planetary Nebula Survey (ChanPlaNS) is a 570 ks Chandra Cycle 12 Large Program targeting 21 high-excitation PNe within ∼1.5 kpc of Earth. When complete, this survey will provide a suite of new X-ray diagnostics that will inform the study of late stellar evolution, binary star astrophysics, and wind interactions. Among the early results of ChanPlaNS (when combined with archival Chandra data) is a surprisingly high detection rate of relatively hard X-ray emission from CSPNe. Specifically, X-ray point sources are clearly detected in roughly half of the ∼30 high-excitation PNe observed thus far by Chandra, and all but one of these X-ray-emitting CSPNe display evidence for a hard (few MK) component in their Chandra spectra. Only the central star of the Dumbbell appears to display 'pure' hot blackbody emission from a ∼200 kK hot white dwarf photosphere in the X-ray band. Potential explanations for the'excess' hard X-ray emission detected from the other CSPNe include late-type companions (heretofore undetected, in most cases) whose coronae have been rejuvenated by recent interactions with the mass-losing WD progenitor, non-LTE effects in hot white dwarf photospheres, self-shocking variable winds from the central star, and slow (re-)accretion of previously ejected red giant envelope mass. © 2012 International Astronomical Union.published_or_final_versio