6,820 research outputs found
Jets and Outflows From Star to Cloud: Observations Confront Theory
In this review we focus on the role jets and outflows play in the star and
planet formation process. Our essential question can be posed as follows: are
jets/outflows merely an epiphenomenon associated with star formation or do they
play an important role in mediating the physics of assembling stars both
individually and globally? We address this question by reviewing the current
state of observations and their key points of contact with theory. Our review
of jet/outflow phenomena is organized into three length-scale domains: Source
and Disk Scales ( au) where the connection with protostellar and disk
evolution theories is paramount; Envelope Scales ( au) where the
chemistry and propagation shed further light on the jet launching process, its
variability and its impact on the infalling envelope; Parent Cloud Scales
( au) where global momentum injection into cluster/cloud
environments become relevant. Issues of feedback are of particular importance
on the smallest scales where planet formation regions in a disk may be impacted
by the presence of disk winds, irradiation by jet shocks or shielding by the
winds. Feedback on envelope scales may determine the final stellar mass
(core-to-star efficiency) and envelope dissipation. Feedback also plays an
important role on the larger scales with outflows contributing to turbulent
support within clusters including alteration of cluster star formation
efficiencies (feedback on larger scales currently appears unlikely). A
particularly novel dimension of our review is that we consider results on jet
dynamics from the emerging field of High Energy Density Laboratory Astrophysics
(HEDLA). HEDLA is now providing direct insights into the 3-D dynamics of fully
magnetized, hypersonic, radiative outflows.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Confirmation of a recent bipolar ejection in the very young hierarchical multiple system IRAS 16293-2422
We present and analyze two new high-resolution (approx 0.3 arcsec),
high-sensitivity (approx 50 uJy beam-1) Very Large Array 3.6 cm observations of
IRAS 16293-2422 obtained in 2007 August and 2008 December. The components
A2alpha and A2beta recently detected in this system are still present, and have
moved roughly symmetrically away from source A2 at a projected velocity of
30-80 km s-1. This confirms that A2alpha and A2beta were formed as a
consequence of a very recent bipolar ejection from A2. Powerful bipolar
ejections have long been known to occur in low-mass young stars, but this is
-to our knowledge-- the first time that such a dramatic one is observed from
its very beginning. Under the reasonable assumption that the flux detected at
radio wavelengths is optically thin free-free emission, one can estimate the
mass of each ejecta to be of the order of 10^-8 Msun. If the ejecta were
created as a consequence of an episode of enhanced mass loss accompanied by an
increase in accretion onto the protostar, then the total luminosity of IRAS
16293-2422 ought to have increased by 10-60% over the course of at least
several months. Between A2alpha and A2beta, component A2 has reappeared, and
the relative position angle between A2 and A1 is found to have increased
significantly since 2003-2005. This strongly suggests that A1 is a protostar
rather than a shock feature, and that the A1/A2 pair is a tight binary system.
Including component B, IRAS 16293-2422 therefore appears to be a very young
hierarchical multiple system.Comment: Accepted for publication in The Astrophysical Journa
The Mass-Velocity and Position-Velocity Relations in Episodic Outflows
While observational evidence for the episodic nature of young stellar
outflows continues to mount, existing numerical and theoretical models of
molecular outflows assume they are formed by the interaction of a non-episodic
wind from a young stellar object with an ambient cloud. In this Letter we
estimate and discuss the effects of episodicity on the mass-velocity and
position-velocity relations observed in molecular outflows. We explain how many
recent observational results disagree with the predictions of non-episodic
outflow models, and we offer simple explanations for the discrepancies. In
particular, we discuss how an episodic stellar outflow can steepen the
power-law slope of the mass-velocity relation in a molecular outflow. And, we
illustrate how an episodic outflow can produce multiple "Hubble-wedges'' in the
position-velocity distribution of a molecular outflow. With a little more
information than we have now, it may be possible to use the "fossil record"
embedded in a molecular outflow's mass-velocity and position-velocity relations
to reconstruct the history of a young stellar object's mass ejection episodes.Comment: To appear in The Astrophysical Journal Letter
Water emission from the chemically rich outflow L1157
In the framework of the Herschel-WISH key program, several ortho-H2O and
para-H2O emission lines, in the frequency range from 500 to 1700 GHz, were
observed with the HIFI instrument in two bow-shock regions (B2 and R) of the
L1157 cloud. The primary aim is to analyse water emission lines as a diagnostic
of the physical conditions in the blue (B2) and red-shifted (R) lobes to
compare the excitation conditions. A total of 5 ortho- and para-H216O plus one
o-H218O transitions were observed in B2 and R with a wide range of excitation
energies (27 K<=Eu<=215 K). The H2O spectra, observed in the two shocked
regions, show that the H2O profiles are markedly different in the two regions.
In particular, at the bow-shock R, we observed broad (~30 km s-1 with respect
to the ambient velocity) red-shifted wings where lines at different excitation
peak at different red-shifted velocities. The B2 spectra are associated with a
narrower velocity range (~6 km s-1), peaking at the systemic velocity. The
excitation analysis suggests, for B2, low values of column density NH2O
<=5{\times}1013 cm-2, a density range of 105 <=nH2 <=107 cm-3, and warm
temperatures (>=300 K). The presence of the broad red-shifted wings and
multiple peaks in the spectra of the R region, prompted the modelling of two
components. High velocities are associated with relatively low temperatures
(~100K),NH2O{\simeq}5{\times}1012-5{\times}1013 cm-2 and densities
nH2{\simeq}106-108 cm-3.Lower velocities are associated with higher excitation
conditions with Tkin>=300 K, very dense gas (nH2 ~108 cm-3) and low column
density (NH2O<5{\times}1013 cm-2).Comment: 9 pages, 8 figures, A&A in pres
Redes de Bases Radiais na prognose de volume de plantaçÔes de Pinus taeda.
O presente trabalho teve como finalidade apresentar uma aplicação da metodo- logia de Redes Neurais Artifi ciais, em especial as Redes de Bases Radiais, para a progno- se do volume final de madeira para plantaçÔes de Pinus taeda, em diferentes condiçÔes impostas no manejo florestal. Com o auxĂlio do software SisPinus, foi possĂvel analisar e identificar os principais atributos para a determinação do volume (24 atributos no total), assim como, auxiliar na geração de 144 instĂąncias a serem utilizadas como inputs (entradas) para a rede gerada pela ferramenta Matlab. A comparação dos dados originais com os simulados apresentou forte convergĂȘncia, alcançando erros quadrĂĄticos extrema- mente baixos para o conjunto de treinamento e para o conjunto de teste, consolidando que a abordagem de Redes de Bases Radiais pode ser uma excelente ferramenta para a prognose de volume da espĂ©cie Pinus taeda, assim como para outras espĂ©cies
Detection of 4765 MHz OH Emission in a Pre-Planetary Nebula -- CRL 618
Jets and outflows are ubiquitous phenomena in astrophysics, found in our
Galaxy in diverse environments, from the formation of stars to late-type
stellar objects. We present observations conducted with the 305m Arecibo
Telescope of the pre-planetary nebula CRL 618 (Westbrook Nebula) - a well
studied late-type star that has developed bipolar jets. The observations
resulted in the first detection of 4765 MHz OH in a late-type stellar object.
The line was narrow (FWHM ~ 0.6 km/s) and ~40 km/s blueshifted with respect to
the systemic velocity, which suggests association with the expanding
jets/bullets in CRL 618. We also report non-detection at Arecibo of any other
OH transition between 1 and 9 GHz. The non-detections were obtained during the
observations in 2008, when the 4765 MHz OH line was first discovered, and also
in 2015 when the 4765 MHz OH line was not detected. Our data indicate that the
4765 MHz OH line was a variable maser. Modeling of the 4765 MHz OH detection
and non-detection of the other transitions is consistent with the physical
conditions expected in CRL 618. The 4765 MHz OH maser could originate from
dissociation of H2O by shocks after sublimation of icy objects in this dying
carbon-rich stellar system, although other alternatives such as OH in an
oxygen-rich circumstellar region associated with a binary companion are also
possible.Comment: 16 pages, 2 figure
The Factory and The Beehive II. Activity and Rotation in Praesepe and the Hyades
Open clusters are collections of stars with a single, well-determined age,
and can be used to investigate the connections between angular-momentum
evolution and magnetic activity over a star's lifetime. We present the results
of a comparative study of the relationship between stellar rotation and
activity in two benchmark open clusters: Praesepe and the Hyades. As they have
the same age and roughly solar metallicity, these clusters serve as an ideal
laboratory for testing the agreement between theoretical and empirical
rotation-activity relations at 600 Myr. We have compiled a sample of
720 spectra --- more than half of which are new observations --- for 516
high-confidence members of Praesepe; we have also obtained 139 new spectra for
130 high-confidence Hyads. We have collected rotation periods () for
135 Praesepe members and 87 Hyads. To compare emission, an indicator
of chromospheric activity, as a function of color, mass, and Rossby number
, we first calculate an expanded set of values, with which we can
obtain the to bolometric luminosity ratio, ,
even when spectra are not flux-calibrated and/or stars lack reliable distances.
Our values cover a broader range of stellar masses and colors (roughly
equivalent to spectral types from K0 to M9), and exhibit better agreement
between independent calculations, than existing values. We find no difference
between the two clusters in their equivalent width or
distributions, and therefore take the merged
and data to be representative of 600-Myr-old stars. Our analysis
shows that activity in these stars is saturated for
. Above that value activity declines as a
power-law with slope , before dropping off rapidly
at ...Comment: 17 pages, 15 figures, Accepted by Ap
The Polarizing Power of the Interstellar Medium in Taurus
We present a study of the polarizing power of the dust in cold dense regions
(dark clouds) compared to that of dust in the general interstellar medium
(ISM). Our study uses new polarimetric, optical, and spectral classification
data for 36 stars to carefully study the relation between polarization
percentage (p) and extinction (A_V) in the Taurus dark cloud complex. We find
two trends in our p-A_V study: (1) stars background to the warm ISM show an
increase in p with A_V; and (2) the percentage of polarization of stars
background to cold dark clouds does not increase with extinction. We detect a
break in the p-A_V relation at an extinction 1.3 +/- 0.2 mag, which we expect
corresponds to a set of conditions where the polarizing power of the dust
associated with the Taurus dark clouds drops precipitously. This breakpoint
places important restrictions on the use of polarimetry in studying
interstellar magnetic fields.Comment: 17 pages, 2 figures, to appear in ApJLett, AASTeX was use
Episodic molecular outflow in the very young protostellar cluster Serpens South
The loss of mass from protostars, in the form of a jet or outflow, is a necessary counterpart to protostellar mass accretion. Outflow ejection events probably vary in their velocity and/or in the rate of mass loss. Such `episodicÂŽ ejection events have been observed during the Class 0 protostellar phase (the early accretion stage), and continue during the subsequent class I phase that marks the first one million years of star formation. Previously observed episodic-ejection sources were relatively isolated; however, the most common sites of star formation are clusters. Outflows link protostars with their environment and provide a viable source of turbulence that is necessary for regulating star formation in clusters, but it is not known how an accretion-driven jet or outflow in a clustered environment manifests itself in its earliest stage. This early stage is important in establishing the initial conditions for momentum and energy transfer to the environment as the protostar and cluster evolve. Here we report that an outflow from a very young class 0 protostar, at the hub of the very active and filamentary Serpens South protostellar cluster, shows unambiguous episodic events. The 12CO (J=2-1) emission from the protostar reveals 22 distinct features of outflow ejecta, the most recent having the highest velocity. The outflow forms bipolar lobes --- one of the first detectable signs of star formation --- which originate from the peak of 1-mm continuum emission. Emission from the surrounding C18O envelope shows kinematics consistent with rotation and an infall of material onto the protostar. The data suggest that episodic accretion-driven outflow begins in the earliest phase of protostellar evolution, and that the outflow remains intact in a very clustered environment, probably providing efficient momentum transfer for driving turbulence. Fil: Plunkett, Adele L. . Yale University. Astronomy Department.; Estados UnidosFil: Arce, HĂ©ctor G.. Yale University. Astronomy Department.; Estados UnidosFil: Mardones, Diego . Universidad de Chile. Departamento de AstronomĂa; ChileFil: van Dokkum, Pieter . Yale University. Astronomy Department.; Estados UnidosFil: Dunham, Michael M. . Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Fernandez Lopez, Manuel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico La Plata. Instituto Argentino de Radioastronomia (i); ArgentinaFil: Gallardo, JosĂ©. Joint ALMA Observatory; ChileFil: Cordero, Stuartt A. . Joint ALMA Observatory; Chil
- âŠ