72 research outputs found
A molecular survey of outflow gas: velocity-dependent shock chemistry and the peculiar composition of the EHV gas
(Abridged) We present a molecular survey of the outflows powered by L1448-mm
and IRAS 04166+2706, two sources with prominent wing and extremely high
velocity (EHV) components in their CO spectra. The molecular composition of the
two outflows presents systematic changes with velocity that we analyze by
dividing the outflow in three chemical regimes, two of them associated with the
wing component and the other the EHV gas. The analysis of the two wing regimes
shows that species like H2CO and CH3OH favor the low-velocity gas, while SiO
and HCN are more abundant in the fastest gas. We also find that the EHV regime
is relatively rich in O-bearing species, as is not only detected in CO and SiO
(already reported elsewhere), but also in SO, CH3OH, and H2CO (newly reported
here), with a tentative detection in HCO+. At the same time, the EHV regime is
relatively poor in C-bearing molecules like CS and HCN. We suggest that this
difference in composition arises from a lower C/O ratio in the EHV gas. The
different chemical compositions of the wing and EHV regimes suggest that these
two outflow components have different physical origins. The wing component is
better explained by shocked ambient gas, although none of the existing shock
models explains all observed features. The peculiar composition of the EHV gas
may reflect its origin as a dense wind from the protostar or its surrounding
disk.Comment: 20 pages, 14 figures. Accepted by Astronomy and Astrophysic
Shells, jets, and internal working surfaces in the molecular outflow from IRAS 04166+2706
Context: IRAS 04166+2706 in Taurus is one of the most nearby young stellar
objects whose molecular outflow contains a highly collimated fast component.
Methods: We have observed the IRAS 04166+2706 outflow with the IRAM Plateau
de Bure interferometer in CO(J=2-1) and SiO(J=2-1) achieving angular
resolutions between 2'' and 4''. To improve the quality of the CO(2-1) images,
we have added single dish data to the interferometer visibilities.
Results: The outflow consists of two distinct components. At velocities <10
km/s, the gas forms two opposed, approximately conical shells that have the YSO
at their vertex. These shells coincide with the walls of evacuated cavities and
seem to result from the acceleration of the ambient gas by a wide-angle wind.
At velocities >30 km/s, the gas forms two opposed jets that travel along the
center of the cavities and whose emission is dominated by a symmetric
collection of at least 7 pairs of peaks. The velocity field of this component
presents a sawtooth pattern with the gas in the tail of each peak moving faster
than the gas in the head. This pattern, together with a systematic widening of
the peaks with distance to the central source, is consistent with the emission
arising from internal working surfaces traveling along the jet and resulting
from variations in the velocity field of ejection. We interpret this component
as the true protostellar wind, and we find its composition consistent with a
chemical model of such type of wind.
Conclusions: Our results support outflow wind models that have simultaneously
wide-angle and narrow components, and suggest that the EHV peaks seen in a
number of outflows consist of internally-shocked wind material.Comment: 13 pages, 10 figures. To appear in A&
Number-phase-squeezed few-photon state generated from squeezed atoms
This paper develops a method of manipulating the squeezed atom state to
generate a few-photon state whose phase or photon-number fluctuations are
prescribed at our disposal. The squeezed atom state is a collective atomic
state whose quantum fluctuations in population difference or collective dipole
are smaller than those of the coherent atom state. It is shown that the
squeezed atom state can be generated by the interaction of atoms with a
coherent state of the electromagnetic field, and that it can be used as a
tunable source of squeezed radiation. A variety of squeezed states, including
the photon-number squeezed state and the phase squeezed state, can be produced
by manipulating the atomic state. This is owing to the fact that
quantum-statistical information of the atomic state is faithfully transferred
to that of the photon state. Possible experimental situations to implement our
theory are discussed.Comment: 17 pages, RevTex, 14 figures, using epsf.sty, title is changed,
discussion about dissipation is added, accepted for publication in Physical
Review
A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system
We present five epochs of VLBI water maser observations around the massive
protostar Cepheus A HW2 with 0.4 mas (0.3 AU) resolution. The main goal of
these observations was to follow the evolution of the remarkable water maser
linear/arcuate structures found in earlier VLBI observations. Comparing the
data of our new epochs of observation with those observed five years before, we
find that at "large" scales of > 1" (700 AU) the main regions of maser emission
persist, implying that both the surrounding medium and the exciting sources of
the masers have been relatively stable during that time span. However, at
smaller scales of < 0.1" (70 AU) we see large changes in the maser structures,
particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly
elliptical patchy ring of ~ 70 mas size (50 AU) with expanding motions of ~ 5
mas/yr (15 km/s). This structure is probably driven by the wind of a still
unidentified YSO located at the centre of the ring (~ 0.18" south of HW2). On
the other hand, the R5 expanding bubble structure (driven by the wind of a
previously identified YSO located ~ 0.6" south of HW2) is currently dissipating
in the circumstellar medium and losing its previous degree of symmetry,
indicating a very short-lived event. In addition, our results reveal, at scales
of ~ 1" (700 AU), the simultaneous presence of a relatively slow (~ 10-70 km/s)
wide-angle outflow (opening angle of ~ 102 deg, traced by the masers, and the
fast (~ 500~km/s) highly collimated radio jet associated with HW2 (opening
angle of ~ 18 deg, previously observed with the VLA. This simultaneous presence
of a wide-angle outflow and a highly collimated jet associated with a massive
protostar is similar to what is found in some low-mass YSOs. The implications
of these results in the study of the formation of high-mass stars are
discussed.Comment: 28 pages, 7 figures. Animations will be included as supporting
material online (MNRAS web page
Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario
We discuss the nuclear magnetic relaxation rate and the superfluid density
with the use of the effective five-band model by Kuroki et al. [Phys. Rev.
Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a
fully-gapped anisotropic \pm s-wave superconductivity consistently explains
experimental observations. In our phenomenological model, the gaps are assumed
to be anisotropic on the electron-like \beta Fermi surfaces around the M point,
where the maximum of the anisotropic gap is about four times larger than the
minimum.Comment: 10 pages, 8 figures; Submitted versio
A SUB-ARCSECOND SURVEY TOWARD CLASS 0 PROTOSTARS IN PERSEUS: SEARCHING FOR SIGNATURES OF PROTOSTELLAR DISKS
Physical Processes in Star Formation
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00693-8.Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.Peer reviewedFinal Accepted Versio
The Physics of Star Cluster Formation and Evolution
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe
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