37 research outputs found
Dense Molecular Gas In A Young Cluster Around MWC 1080 -- Rule Of The Massive Star
We present CS , CO , and CO , observations with the 10-element Berkeley Illinois Maryland Association
(BIMA) Array toward the young cluster around the Be star MWC 1080. These
observations reveal a biconical outflow cavity with size 0.3 and 0.05 pc
for the semimajor and semiminor axis and 45\arcdeg position angle.
These transitions trace the dense gas, which is likely the swept-up gas of the
outflow cavity, rather than the remaining natal gas or the outflow gas. The gas
is clumpy; thirty-two clumps are identified. The identified clumps are
approximately gravitationally bound and consistent with a standard isothermal
sphere density, which suggests that they are likely collapsing protostellar
cores. The gas kinematics suggests that there exists velocity gradients
implying effects from the inclination of the cavity and MWC 1080. The
kinematics of dense gas has also been affected by either outflows or stellar
winds from MWC 1080, and lower-mass clumps are possibly under stronger effects
from MWC 1080 than higher-mass clumps. In addition, low-mass cluster members
tend to be formed in the denser and more turbulent cores, compared to isolated
low-mass star-forming cores. This results from contributions of nearby forming
massive stars, such as outflows or stellar winds. Therefore, we conclude that
in clusters like the MWC 1080 system, effects from massive stars dominate the
star-forming environment in both the kinematics and dynamics of the natal cloud
and the formation of low-mass cluster members. This study provides insights
into the effects of MWC 1080 on its natal cloud, and suggests a different
low-mass star forming environment in clusters compared to isolated star
formation.Comment: 42 pages, 5 tables, and 13 figures, accepted for publication in Ap
Mid-infrared imaging of the massive young star AFGL 2591: Probing the circumstellar environment of an outflow source
Most, if not all, stars are now believed to produce energetic outflows during
their formation. Yet, almost 20 years after the discovery of bipolar outflows
from young stars, the origins of this violent phenomenon are not well
understood. One of the difficulties of probing the outflow process,
particularly in the case of massive embedded stars, is a deficit of high
spatial resolution observations. Here, we present sub-arcsecond-resolution
mid-infrared images of one massive young stellar object, AFGL 2591, and its
immediate surroundings. Our images, at 11.7, 12.5 and 18.0 microns, reveal a
knot of emission ~6'' SW of the star, which may be evidence for a recent
ejection event or an embedded companion star. This knot is roughly coincident
with a previously seen near-infrared reflection nebula and a radio source, and
lies within the known large-scale CO outflow. We also find a new faint NW
source which may be another embedded lower-luminosity star. The IRAS
mid-infrared spectrum of AFGL 2591 shows a large silicate absorption feature at
10 microns, implying that the primary source is surrounded by an optically
thick dusty envelope. We discuss the interrelationship of these phenomena and
suggest that mid-infrared imaging and spectroscopy provide powerful tools for
probing massive star birth.Comment: 14 pages, 3 PostScript figures, accepted for publication in The
Astrophysical Journal Letter
An equatorial wind from the massive young stellar object S140 IRS 1
The discovery of the second equatorial ionized stellar wind from a massive
young stellar object is reported. High resolution radio continuum maps of S140
IRS 1 reveal a highly elongated source that is perpendicular to the larger
scale bipolar molecular outflow. This picture is confirmed by location of a
small scale monopolar near-IR reflection nebula at the base of the blueshifted
lobe. A second epoch of observations over a five year baseline show little
ordered outward proper motion of clumps as would have been expected for a jet.
A third epoch, taken only 50 days after the second, did show significant
changes in the radio morphology. These radio properties can all be understood
in the context of an equatorial wind driven by radiation pressure from the
central star and inner disc acting on the gas in the surface layers of the disc
as proposed by Drew et al. (1998). This equatorial wind system is briefly
compared with the one in S106IR, and contrasted with other massive young
stellar objects that drive ionized jets.Comment: 19 pages, 5 figures, accepted by ApJ, minor changes in light of
referees repor
The Optical Proper Motions of HH 7-11 and Cep E (HH 377)
A key ingredient in understanding the dynamics of stellar outflows is their
proper motion. We have used optical images in the [SII] emission at 6717/31 A
and the red Digitized Palomar Observatory Sky Survey (DSS) plates to determine
the proper motion of HH 7-11 system and the optical knot of Cep E (HH 377). The
DSS plate measurements span nearly 37 years for both HH 7-11 and HH 377 and
have wide field of view, which allows an accurate determination of the proper
motions despite their relatively low angular resolution. The optical images,
with higher angular resolution, cover a shorter period of 7 and 4 years,
respectively, and have been used to complement the DSS measurements. From the
DSS plates we have found that HH 377 has a proper motion of 0.031 +/- 0.003
arcsec/yr with a PA = 206 arcdeg, i.e. moving away from IRAS 230111+63, that at
a distance of 730 pc corresponds to a tangential velocity of 107 +/- 14 km/s.
The values obtained from the optical images are consistent with these
measurements. Similarly, the proper motions of HH 7-11 range from 0.015 +/-
0.009 (HH 9) to 0.044 +/- 0.007 (HH 11) arcsec/yr, and the flow is moving away
from SVS 13 with a mean PA = 136 arcdeg. At a distance of 330 pc, these motions
correspond to tangential velocities of 25 - 70 km/s, i.e. comparable to the
original values obtained by Herbig & Jones (1983). The measurements from the
optical CCD [SII] images are again consistent with these motions, although in
detail there are some difference, particularly for HH 7 and HH 10.Comment: 18 pages, 6 Figures (jpgs because of their size
The X-ray emission from Z CMa during an FUor-like outburst and the detection of its X-ray jet
Accretion shocks have been recognized as important X-ray emission mechanism
for pre-main sequence stars. Yet the X-ray properties of FUor outbursts, events
that are caused by violent accretion, have been given little attention. We have
observed the FUor object Z CMa during optical outburst and quiescence with
Chandra. No significant changes in X-ray brightness and spectral shape are
found, suggesting that the X-ray emission is of coronal nature. Due to the
binary nature of Z CMa the origin of the X-ray source is ambiguous. However,
the moderate hydrogen column density derived from our data makes it unlikely
that the embedded primary star is the X-ray source. The secondary star, which
is the FUor object, is thus responsible for both the X-ray emission and the
presently ongoing accretion outburst, which seem however to be unrelated
phenomena. The secondary is also known to drive a large outflow and jet, that
we detect here for the first time in X-rays. The distance of the X-ray emitting
outflow source to the central star is higher than in jets of low-mass stars.Comment: 5 pages, accepted for publication in Astronomy & Astrophysic
Polarimetric variations of binary stars. III Periodic polarimetric variations of the Herbig Ae/Be star MWC 1080
We present polarimetric observations of a massive pre-main sequence
short-period binary star of the Herbig Ae/Be type, MWC 1080. The mean
polarization at 7660 A is 1.60% at 81.6 deg, or 0.6% at 139 deg if an estimate
of the interstellar polarization is subtracted. The intrinsic polarization
points to an asymmetric geometry of the circumstellar or circumbinary
environment while the 139 deg intrinsic position angle traces the axis of
symmetry of the system and is perpendicular to the position angle of the
outflow cavity. The polarization and its position angle are clearly variable,
at all wavelengths, and on time scales of hours, days, months, and years.
Stochastic variability is accompanied by periodic variations caused by the
orbital motion of the stars in their dusty environment. These periodic
polarimetric variations are the first phased-locked ones detected for a
pre-main sequence binary. The variations are not simply double-periodic (seen
twice per orbit) but include single-periodic (seen once per orbit) and
higher-order variations. The presence of single-periodic variations could be
due to non equal mass stars, the presence of dust grains, an asymmetric
configuration of the circumstellar or circumbinary material, or the
eccentricity of the orbit. MWC 1080 is an eclipsing binary with primary and
secondary eclipses occurring at phases 0.0 and 0.55. The signatures of the
eclipses are seen in the polarimetric observations.Comment: 30 pages, 8 figures, to be published in the Astronomical Journa
The Stellar Composition of the Star Formation Region CMa R1 -- III. A new outburst of the Be star component in Z CMa
We report on a recent event in which, after more than a decade of slowly
fading, the visual brightness of the massive young binary Z CMa suddenly
started to rise by about 1 magnitude in December 1999, followed by a rapid
decline to its previous brightness over the next six months. This behaviour is
similar to that exhibited by this system around its eruption in February 1987.
A comparison of the intrinsic luminosities of the system with recent
evolutionary calculations shows that Z CMa may consist of a 16 M_sun B0 IIIe
primary star and a ~ 3 M_sun FUOr secondary with a common age of ~ 3 x 10^5 yr.
We also compare new high-resolution spectra obtained in Jan. and Feb. 2000,
during the recent rise in brightness, with archive data from 1991 and 1996. The
spectra are rich in emission lines, which originate from the envelope of the
early B-type primary star. The strength of these emission lines increased
strongly with the brightness of Z CMa. We interpret the collected spectral data
in terms of an accretion disc with atmosphere around the Herbig B0e component
of Z CMa, which has expanded during the outbursts of 1987 and 2000. A high
resolution profile of the 6300 A [O I] emission line, obtained by us in March
2002 shows an increase in flux and a prominent blue shoulder to the feature
extending to ~ -700 km/s, which was much fainter in the pre-outburst spectra.
We propose that this change in profile is a result of a strong change in the
collimation of a jet, as a result of the outburst at the start of this century.Comment: 22 pages, 12 figures, accepted for publication in MNRA
Clustered star formation and outflows in AFGL 2591
We report on a detailed study of the water maser kinematics and radio
continuum emission toward the most massive and young object in the star-forming
region AFGL 2591. Our analysis shows at least two spatial scales of multiple
star formation, one projected across 0.1 pc on the sky and another one at about
2000 AU from a ZAMS star of about 38 Solar masses. This young stellar object
drives a powerful jet- and wind-driven outflow system with the water masers
associated to the outflow walls, previously detected as a limb-brightened
cavity in the NIR band. At about 1300 AU to the north of this object a younger
protostar drives two bow shocks, outlined by arc-like water maser emission, at
200 AU either side of the source. We have traced the velocity profile of the
gas that expands along these arc-like maser structures and compared it with the
jet-driven outflow model. This analysis suggests that the ambient medium around
the northern protostar is swept up by a jet-driven shock (>66 km/s) and perhaps
a lower-velocity (~10 km/s) wind with an opening angle of about 20 degrees from
the jet axis.Comment: 21 pages, 4 figures, 2 tables, accepted by The Astrophysical Journa
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
The Balloon-Borne Large Aperture Submillimeter Telescope (BLAST) 2005: A 10 deg^2 Survey of Star Formation in Cygnus X
We present Cygnus X in a new multi-wavelength perspective based on an
unbiased BLAST survey at 250, 350, and 500 micron, combined with rich datasets
for this well-studied region. Our primary goal is to investigate the early
stages of high mass star formation. We have detected 184 compact sources in
various stages of evolution across all three BLAST bands. From their
well-constrained spectral energy distributions, we obtain the physical
properties mass, surface density, bolometric luminosity, and dust temperature.
Some of the bright sources reaching 40 K contain well-known compact H II
regions. We relate these to other sources at earlier stages of evolution via
the energetics as deduced from their position in the luminosity-mass (L-M)
diagram. The BLAST spectral coverage, near the peak of the spectral energy
distribution of the dust, reveals fainter sources too cool (~ 10 K) to be seen
by earlier shorter-wavelength surveys like IRAS. We detect thermal emission
from infrared dark clouds and investigate the phenomenon of cold ``starless
cores" more generally. Spitzer images of these cold sources often show stellar
nurseries, but these potential sites for massive star formation are ``starless"
in the sense that to date there is no massive protostar in a vigorous accretion
phase. We discuss evolution in the context of the L-M diagram. Theory raises
some interesting possibilities: some cold massive compact sources might never
form a cluster containing massive stars; and clusters with massive stars might
not have an identifiable compact cold massive precursor.Comment: 42 pages, 31 Figures, 6 table