23 research outputs found
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
APEX-CHAMP+ high-J CO observations of low-mass young stellar objects: III. NGC 1333 IRAS 4A/4B envelope, outflow and UV heating
NGC 1333 IRAS 4A and IRAS 4B sources are among the best studied Stage 0
low-mass protostars which are driving prominent bipolar outflows. Most studies
have so far concentrated on the colder parts (T<30K) of these regions. The aim
is to characterize the warmer parts of the protostellar envelope in order to
quantify the feedback of the protostars on their surroundings in terms of
shocks, UV heating, photodissociation and outflow dispersal. Fully sampled
large scale maps of the region were obtained; APEX-CHAMP+ was used for 12CO
6-5, 13CO 6-5 and [CI] 2-1, and JCMT-HARP-B for 12CO 3-2 emissions.
Complementary Herschel-HIFI and ground-based lines of CO and its isotopologs,
from 1-0 upto 10-9 (Eu/k 300K), are collected at the source positions.
Radiative-transfer models of the dust and lines are used to determine
temperatures and masses of the outflowing and UV-heated gas and infer the CO
abundance structure. Broad CO emission line profiles trace entrained shocked
gas along the outflow walls, with typical temperatures of ~100K. At other
positions surrounding the outflow and the protostar, the 6-5 line profiles are
narrow indicating UV excitation. The narrow 13CO 6-5 data directly reveal the
UV heated gas distribution for the first time. The amount of UV-heated and
outflowing gas are found to be comparable from the 12CO and 13CO 6-5 maps,
implying that UV photons can affect the gas as much as the outflows. Weak [CI]
emission throughout the region indicates a lack of CO dissociating photons.
Modeling of the C18O lines indicates the necessity of a "drop" abundance
profile throughout the envelope where the CO freezes out and is reloaded back
into the gas phase, thus providing quantitative evidence for the CO ice
evaporation zone around the protostars. The inner abundances are less than the
canonical value of CO/H_2=2.7x10^-4, indicating some processing of CO into
other species on the grains.Comment: 20 pages, 22 figures, Accepted by A&
APEX-CHAMP+ high-J CO observations of low-mass young stellar objects: IV. Mechanical and radiative feedback
During the embedded stage of star formation, bipolar molecular outflows and
UV radiation from the protostar are important feedback processes. Our aim is to
quantify the feedback, mechanical and radiative, for a large sample of low-mass
sources. The outflow activity is compared to radiative feedback in the form of
UV heating by the accreting protostar to search for correlations and
evolutionary trends. Large-scale maps of 26 young stellar objects, which are
part of the Herschel WISH key program are obtained using the CHAMP+ instrument
on the APEX (12CO and 13CO 6-5), and the HARP-B instrument on the JCMT (12CO
and 13CO 3-2). Maps are used to determine outflow parameters and envelope
models are used to quantify the amount of UV-heated gas and its temperature
from 13CO 6-5 observations. All sources in our sample show outflow activity and
the outflow force, F_CO, is larger for Class 0 sources than for Class I
sources, even if their luminosities are comparable. The outflowing gas
typically extends to much greater distances than the power-law envelope and
therefore influences the surrounding cloud material directly. Comparison of the
CO 6-5 results with Herschel-HIFI H2O and PACS high-J CO lines, both tracing
currently shocked gas, shows that the two components are linked, even though
the transitions do not probe the same gas. The link does not extend down to CO
3-2. The conclusion is that CO 6-5 depends on the shock characteristics
(density and velocity), whereas CO 3-2 is more sensitive to conditions in the
surrounding environment (density). The radiative feedback is responsible for
increasing the gas temperature by a factor of two, up to 30-50 K, on scales of
a few thousand AU, particularly along the direction of the outflow. The mass of
the UV heated gas exceeds the mass contained in the entrained outflow in the
inner ~3000 AU and is therefore at least as important on small scales.Comment: 30 pages with Appendix, Accepted by Astronomy & Astrophysic
First detection of gas-phase ammonia in a planet-forming disk. NH₃, N₂H⁺, and H₂O in the disk around TW Hydrae
Context. Nitrogen chemistry in protoplanetary disks and the freeze-out on dust particles is key for understanding the formation of nitrogen-bearing species in early solar system analogs. In dense cores, 10% to 20% of the nitrogen reservoir is locked up in ices such as NH3, NH4+ and OCN−. So far, ammonia has not been detected beyond the snowline in protoplanetary disks. Aims. We aim to find gas-phase ammonia in a protoplanetary disk and characterize its abundance with respect to water vapor. Methods. Using HIFI on the Herschel Space Observatory, we detected for the first time the ground-state rotational emission of ortho-NH3 in a protoplanetary disk around TW Hya. We used detailed models of the disk’s physical structure and the chemistry of ammonia and water to infer the amounts of gas-phase molecules of these species. We explored two radial distributions (extended across the disk and confined to <60 au like the millimeter-sized grains) and two vertical distributions (near the midplane and at intermediate heights above the midplane, where water is expected to photodesorb off icy grains) to describe the (unknown) location of the molecules. These distributions capture the effects of radial drift and vertical settling of ice-covered grains. Results. The NH310–00 line is detected simultaneously with H2O 110–101 at an antenna temperature of 15.3 mK in the Herschel beam; the same spectrum also contains the N2H+ 6–5 line with a strength of 18.1 mK. We use physical-chemical models to reproduce the fluxes and assume that water and ammonia are cospatial. We infer ammonia gas-phase masses of 0.7−11.0 × 1021 g, depending on the adopted spatial distribution, in line with previous literature estimates. For water, we infer gas-phase masses of 0.2−16.0 × 1022 g, improving upon earlier literature estimates This corresponds to NH3/H2O abundance ratios of 7%−84%, assuming that water and ammonia are co-located. The inferred N2H+ gas mass of 4.9 × 1021 g agrees well with earlier literature estimates that were based on lower excitation transitions. These masses correspond to a disk-averaged abundances of 0.2−17.0 × 10-11, 0.1−9.0 × 10-10 and 7.6 × 10-11 for NH3, H2O and N2H+ respectively. Conclusions. Only in the most compact and settled adopted configuration is the inferred NH3/H2O consistent with interstellar ices and solar system bodies of ~5%–10%; all other spatial distributions require additional gas-phase NH3 production mechanisms. Volatile release in the midplane may occur through collisions between icy bodies if the available surface for subsequent freeze-out is significantly reduced, for instance, through growth of small grains into pebbles or larger bodies