631 research outputs found
Survival of molecular gas in cavities of transition disks (I. CO)
(Abridged) Planet formation is closely related to the structure and dispersal
of protoplanetary disks. A certain class of disks, called transition disks,
exhibit cavities in dust images at scales of up to a few 10s of AU. The
formation mechanism of the cavities is still unclear. The gas content of such
cavities can be spatially resolved for the first time using the Atacama Large
Millimeter/submillimeter Array (ALMA).
We have developed a new series of models to simulate the physical conditions
and chemical abundances of the gas in cavities to address the question whether
the gas is primarily atomic or molecular inside the dust free cavities exposed
to intense UV radiation. Molecular/atomic line emission by carbon monoxide
(CO), its isotopologues (13CO, C18O, C17O, and 13C18O) and related species
([CI], [CII], and [OI]) is predicted for comparison with ALMA and the Herschel
Space Observatory.
The gas can remain in molecular form down to very low amounts of gas in the
cavity. Shielding of the stellar radiation by a dusty inner disk
(pre-transition disk) allows CO to survive down to lower gas masses in the
cavity. The main parameter for the CO emission from cavity is the gas mass.
Other parameters such as the outer disk mass, bolometric luminosity, shape of
the stellar spectrum or PAH abundance are less important. Since the CO pure
rotational lines readily become optically thick, the CO isotopologues need to
be observed in order to quantitatively determine the amount of gas in the
cavity.
A wide range of gas masses in the cavity of transition disks (~4 orders of
magnitude) can be probed using combined observations of CO isotopologue lines
with ALMA. Measuring the gas mass in the cavity will ultimately help to
distinguish between different cavity formation theories.Comment: Accepted by A&A, 21 pages, 20 figure
Herschel/HIFI detections of hydrides towards AFGL 2591: Envelope emission versus tenuous cloud absorption
The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory allows the first observations of light diatomic
molecules at high spectral resolution and in multiple transitions. Here, we report deep integrations using HIFI in different lines of hydrides
towards the high-mass star forming region AFGL 2591. Detected are CH, CH^+, NH, OH^+, H_2O^+, while NH^+ and SH^+ have not been detected. All
molecules except for CH and CH^+ are seen in absorption with low excitation temperatures and at velocities different from the systemic velocity
of the protostellar envelope. Surprisingly, the CH(J_(F,P) = 3/2_(2,−) − 1/2_(1,+)) and CH^+(J = 1−0, J = 2−1) lines are detected in emission at the
systemic velocity. We can assign the absorption features to a foreground cloud and an outflow lobe, while the CH and CH^+ emission stems from
the envelope. The observed abundance and excitation of CH and CH^+ can be explained in the scenario of FUV irradiated outflow walls, where
a cavity etched out by the outflow allows protostellar FUV photons to irradiate and heat the envelope at larger distances driving the chemical
reactions that produce these molecules
Exploiting boundary states of imperfect spin chains for high-fidelity state transfer
We study transfer of a quantum state through XX spin chains with static
imperfections. We combine the two standard approaches for state transfer based
on (i) modulated couplings between neighboring spins throughout the spin chain
and (ii) weak coupling of the outermost spins to an unmodulated spin chain. The
combined approach allows us to design spin chains with modulated couplings and
localized boundary states, permitting high-fidelity state transfer in the
presence of random static imperfections of the couplings. The modulated
couplings are explicitly obtained from an exact algorithm using the close
relation between tridiagonal matrices and orthogonal polynomials [Linear
Algebr. Appl. 21, 245 (1978)]. The implemented algorithm and a graphical user
interface for constructing spin chains with boundary states (spinGUIn) are
provided as Supplemental Material.Comment: 7 pages, 3 figures + spinGUIn description and Matlab files
iepsolve.m, spinGUIn.fig, spinGUIn.
CN rings in full protoplanetary disks around young stars as probes of disk structure
Bright ring-like structure emission of the CN molecule has been observed in
protoplanetary disks. We investigate whether such structures are due to the
morphology of the disk itself or if they are instead an intrinsic feature of CN
emission. With the intention of using CN as a diagnostic, we also address to
which physical and chemical parameters CN is most sensitive. A set of disk
models were run for different stellar spectra, masses, and physical structures
via the 2D thermochemical code DALI. An updated chemical network that accounts
for the most relevant CN reactions was adopted. Ring-shaped emission is found
to be a common feature of all adopted models; the highest abundance is found in
the upper outer regions of the disk, and the column density peaks at 30-100 AU
for T Tauri stars with standard accretion rates. Higher mass disks generally
show brighter CN. Higher UV fields, such as those appropriate for T Tauri stars
with high accretion rates or for Herbig Ae stars or for higher disk flaring,
generally result in brighter and larger rings. These trends are due to the main
formation paths of CN, which all start with vibrationally excited H2*
molecules, that are produced through far ultraviolet (FUV) pumping of H2. The
model results compare well with observed disk-integrated CN fluxes and the
observed location of the CN ring for the TW Hya disk. CN rings are produced
naturally in protoplanetary disks and do not require a specific underlying disk
structure such as a dust cavity or gap. The strong link between FUV flux and CN
emission can provide critical information regarding the vertical structure of
the disk and the distribution of dust grains which affects the UV penetration,
and could help to break some degeneracies in the SED fitting. In contrast with
C2H or c-C3H2, the CN flux is not very sensitive to carbon and oxygen
depletion.Comment: New version of paper, correcting too high H2 excitation rates and
consequently too high CN column densities. Qualitative conclusions of the
paper remain unchanged. Quantitatively, the CN column densities are an order
of magnitude lower whereas fluxes decrease by a factor of 3-4. Rings are
larger by up to a factor of 2. 13 pages, 19 figures, accepted for publication
in A&
Depletion of molecular gas by an accretion outburst in a protoplanetary disk
We investigate new and archival 3-5 m high resolution ( km
s) spectroscopy of molecular gas in the inner disk of the young
solar-mass star EX Lupi, taken during and after the strong accretion outburst
of 2008. The data were obtained using the CRIRES spectrometer at the ESO Very
Large Telescope in 2008 and 2014. In 2008, emission lines from CO, HO,
and OH were detected with broad profiles tracing gas near and within the
corotation radius (0.02-0.3 AU). In 2014, the spectra display marked
differences. The CO lines, while still detected, are much weaker, and the
HO and OH lines have disappeared altogether. At 3 m a veiled stellar
photospheric spectrum is observed. Our analysis finds that the molecular gas
mass in the inner disk has decreased by an order of magnitude since the
outburst, matching a similar decrease in the accretion rate onto the star. We
discuss these findings in the context of a rapid depletion of material
accumulated beyond the disk corotation radius during quiescent periods, as
proposed by models of episodic accretion in EXor type young stars.Comment: 6 pages, 4 figures, 1 table, accepted for publication in the
Astrophysical Journal Letter
Breathing oscillations of a trapped impurity in a Bose gas
Motivated by a recent experiment [J. Catani et al., arXiv:1106.0828v1
preprint, 2011], we study breathing oscillations in the width of a harmonically
trapped impurity interacting with a separately trapped Bose gas. We provide an
intuitive physical picture of such dynamics at zero temperature, using a
time-dependent variational approach. In the Gross-Pitaevskii regime we obtain
breathing oscillations whose amplitudes are suppressed by self trapping, due to
interactions with the Bose gas. Introducing phonons in the Bose gas leads to
the damping of breathing oscillations and non-Markovian dynamics of the width
of the impurity, the degree of which can be engineered through controllable
parameters. Our results reproduce the main features of the impurity dynamics
observed by Catani et al. despite experimental thermal effects, and are
supported by simulations of the system in the Gross-Pitaevskii regime.
Moreover, we predict novel effects at lower temperatures due to self-trapping
and the inhomogeneity of the trapped Bose gas.Comment: 7 pages, 3 figure
Analytical Approximations for the Collapse of an Empty Spherical Bubble
The Rayleigh equation 3/2 R'+RR"+p/rho=0 with initial conditions R(0)=Rmax,
R'(0)=0 models the collapse of an empty spherical bubble of radius R(T) in an
ideal, infinite liquid with far-field pressure p and density rho. The solution
for r=R/Rmax as a function of time t=T/Tcollapse, where R(Tcollapse)=0, is
independent of Rmax, p, and rho. While no closed-form expression for r(t) is
known we find that s(t)=(1-t^2)^(2/5) approximates r(t) with an error below 1%.
A systematic development in orders of t^2 further yields the
0.001%-approximation r*(t)=s(t)[1-a Li(2.21,t^2)], where a=-0.01832099 is a
constant and Li is the polylogarithm. The usefulness of these approximations is
demonstrated by comparison to high-precision cavitation data obtained in
microgravity.Comment: 5 pages, 2 figure
Disentangling the jet emission from protostellar systems. The ALMA view of VLA1623
Context: High-resolution studies of class 0 protostars represent the key to
constraining protostar formation models. VLA16234-2417 represents the prototype
of class 0 protostars, and it has been recently identified as a triple
non-coeval system. Aim: We aim at deriving the physical properties of the jets
in VLA16234-2417 using tracers of shocked gas. Methods: ALMA Cycle 0 Early
Science observations of CO(2-1) in the extended configuration are presented in
comparison with previous SMA CO(3-2) and Herschel-PACS [OI}] 63 micron
observations. Gas morphology and kinematics were analysed to constrain the
physical structure and origin of the protostellar outflows. Results: We reveal
a collimated jet component associated with the [OI] 63 micron emission at about
8'' (about 960 AU) from source B. This newly detected jet component is
inversely oriented with respect to the large-scale outflow driven by source A,
and it is aligned with compact and fast jet emission very close to source B
(about 0.3'') rather than with the direction perpendicular to the A disk. We
also detect a cavity-like structure at low projected velocities, which
surrounds the [OI] 63 micron emission and is possibly associated with the
outflow driven by source A. Finally, no compact outflow emission is associated
with source W. Conclusions: Our high-resolution ALMA observations seem to
suggest there is a fast and collimated jet component associated with source B.
This scenario would confirm that source B is younger than A, that it is in a
very early stage of evolution, and that it drives a faster, more collimated,
and more compact jet with respect to the large-scale slower outflow driven by
A. However, a different scenario of a precessing jet driven by A cannot be
firmly excluded from the present observations.Comment: Accepted for publication in Astronomy & Astrophysic
GMC Collisions as Triggers of Star Formation. I. Parameter Space Exploration with 2D Simulations
We utilize magnetohydrodynamic (MHD) simulations to develop a numerical model for GMC-GMC collisions between nearly magnetically critical clouds. The goal is to determine if, and under what circumstances, cloud collisions can cause pre-existing magnetically subcritical clumps to become supercritical and undergo gravitational collapse. We first develop and implement new photodissociation region (PDR) based heating and cooling functions that span the atomic to molecular transition, creating a multiphase ISM and allowing modeling of non-equilibrium temperature structures. Then in 2D and with ideal MHD, we explore a wide parameter space of magnetic field strength, magnetic field geometry, collision velocity, and impact parameter, and compare isolated versus colliding clouds. We find factors of ~2-3 increase in mean clump density from typical collisions, with strong dependence on collision velocity and magnetic field strength, but ultimately limited by flux-freezing in 2D geometries. For geometries enabling flow along magnetic field lines, greater degrees of collapse are seen. We discuss observational diagnostics of cloud collisions, focussing on 13CO(J=2-1), 13CO(J=3-2), and 12CO(J=8-7) integrated intensity maps and spectra, which we synthesize from our simulation outputs. We find the ratio of J=8-7 to lower-J emission is a powerful diagnostic probe of GMC collisions
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