48 research outputs found
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&
Warm formaldehyde in the Oph IRS 48 transitional disk
Simple molecules like H2CO and CH3OH in protoplanetary disks are the starting
point for the production of more complex organic molecules. So far, the
observed chemical complexity in disks has been limited due to freeze out of
molecules onto grains in the bulk of the cold outer disk. Complex molecules can
be studied more directly in transitional disks with large inner holes, as these
have a higher potential of detection, through UV heating of the outer disk and
the directly exposed midplane at the wall. We use Atacama Large
Millimeter/submillimeter Array (ALMA) Band 9 (~680 GHz) line data of the
transitional disk Oph IRS 48, previously shown to have a large dust trap, to
search for complex molecules in regions where planetesimals are forming. We
report the detection of the H2CO 9(1,8)-8(1,7) line at 674 GHz, which is
spatially resolved as a semi-ring at ~60 AU radius centered south from the
star. The inferred H2CO abundance is ~10^{-8} derived by combining a physical
disk model of the source with a non-LTE excitation calculation. Upper limits
for CH3OH lines in the same disk give an abundance ratio H2CO/CH3OH>0.3, which
points to both ice formation and gas-phase routes playing a role in the H2CO
production. Upper limits on the abundances of H13CO+, CN and several other
molecules in the disk are also derived and found to be consistent with full
chemical models. The detection of the H2CO line demonstrates the start of
complex organic molecules in a planet-forming disk. Future ALMA observations
should be able to push down the abundance detection limits of other molecules
by 1-2 orders of magnitude and test chemical models of organic molecules in
(transitional) disks.Comment: Updated references and minor changes to text, approved by language
edito
A Major Asymmetric Dust Trap in a Transition Disk
The statistics of discovered exoplanets suggest that planets form
efficiently. However, there are fundamental unsolved problems, such as
excessive inward drift of particles in protoplanetary disks during planet
formation. Recent theories invoke dust traps to overcome this problem. We
report the detection of a dust trap in the disk around the star Oph IRS 48
using observations from the Atacama Large Millimeter/submillimeter Array
(ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast
crescent-shaped emission on one side of the star originating from
millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized
dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest
rings centered on the star. The difference in distribution of big grains versus
small grains/gas can be modeled with a vortex-shaped dust trap triggered by a
companion.Comment: 25 pages, 7 figures (accepted version prior to language editing
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
CO in Protostars (COPS): -SPIRE Spectroscopy of Embedded Protostars
We present full spectral scans from 200-670m of 26 Class 0+I
protostellar sources, obtained with -SPIRE, as part of the
"COPS-SPIRE" Open Time program, complementary to the DIGIT and WISH Key
programs. Based on our nearly continuous, line-free spectra from 200-670
m, the calculated bolometric luminosities () increase by 50%
on average, and the bolometric temperatures () decrease by 10% on
average, in comparison with the measurements without Herschel. Fifteen
protostars have the same Class using and /. We identify rotational transitions of CO lines from J=4-3 to J=13-12,
along with emission lines of CO, HCO, HO, and [CI]. The ratios
of CO to CO indicate that CO emission remains optically
thick for < 13. We fit up to four components of temperature from
the rotational diagram with flexible break points to separate the components.
The distribution of rotational temperatures shows a primary population around
100 K with a secondary population at 350 K. We quantify the correlations
of each line pair found in our dataset, and find the strength of correlation of
CO lines decreases as the difference between -level between two CO lines
increases. The multiple origins of CO emission previously revealed by
velocity-resolved profiles are consistent with this smooth distribution if each
physical component contributes to a wide range of CO lines with significant
overlap in the CO ladder. We investigate the spatial extent of CO emission and
find that the morphology is more centrally peaked and less bipolar at high-
lines. We find the CO emission observed with SPIRE related to outflows, which
consists two components, the entrained gas and shocked gas, as revealed by our
rotational diagram analysis as well as the studies with velocity-resolved CO
emission.Comment: 50 pages, 18 figures, accepted to ApJS. Revised for Table 6 and
Figure
An Analysis of the Environments of FU Orionis Objects with Herschel
We present Herschel-HIFI, SPIRE, and PACS 50-670 {\mu}m imaging and
spectroscopy of six FU Orionis-type objects and candidates (FU Orionis, V1735
Cyg, V1515 Cyg, V1057 Cyg, V1331 Cyg, and HBC 722), ranging in outburst date
from 1936-2010, from the "FOOSH" (FU Orionis Objects Surveyed with Herschel)
program, as well as ancillary results from Spitzer-IRS and the Caltech
Submillimeter Observatory. In their system properties (Lbol, Tbol, line
emission), we find that FUors are in a variety of evolutionary states.
Additionally, some FUors have features of both Class I and II sources: warm
continuum consistent with Class II sources, but rotational line emission
typical of Class I, far higher than Class II sources of similar
mass/luminosity. Combining several classification techniques, we find an
evolutionary sequence consistent with previous mid-IR indicators. We detect [O
I] in every source at luminosities consistent with Class 0/I protostars, much
greater than in Class II disks. We detect transitions of 13CO (J_up of 5 to 8)
around two sources (V1735 Cyg and HBC 722) but attribute them to nearby
protostars. Of the remaining sources, three (FU Ori, V1515 Cyg, and V1331 Cyg)
exhibit only low-lying CO, but one (V1057 Cyg) shows CO up to J = 23 - 22 and
evidence for H2O and OH emission, at strengths typical of protostars rather
than T Tauri stars. Rotational temperatures for "cool" CO components range from
20-81 K, for ~ 10^50 total CO molecules. We detect [C I] and [N II] primarily
as diffuse emission.Comment: 31 pages, 15 figures; accepted to Ap
Revealing The Millimeter Environment of the New FU Orionis Candidate HBC722 with the Submillimeter Array
We present 230 GHz Submillimeter Array continuum and molecular line
observations of the newly discovered FUor candidate HBC722. We report the
detection of seven 1.3 mm continuum sources in the vicinity of HBC722, none of
which correspond to HBC722 itself. We compile infrared and submillimeter
continuum photometry of each source from previous studies and conclude that
three are Class 0 embedded protostars, one is a Class I embedded protostar, one
is a Class I/II transition object, and two are either starless cores or very
young, very low luminosity protostars or first hydrostatic cores. We detect a
northwest-southeast outflow, consistent with the previous detection of such an
outflow in low-resolution, single-dish observations, and note that its axis may
be precessing. We show that this outflow is centered on and driven by one of
the nearby Class 0 sources rather than HBC722, and find no conclusive evidence
that HBC722 itself is driving an outflow. The non-detection of HBC722 in the
1.3 mm continuum observations suggests an upper limit of 0.02 solar masses for
the mass of the circumstellar disk. This limit is consistent with typical T
Tauri disks and with a disk that provides sufficient mass to power the burst.Comment: 12 pages, 7 figures, accepted by Ap
Disk Imaging Survey of Chemistry with SMA (DISCS): I. Taurus Protoplanetary Disk Data
Chemistry plays an important role in the structure and evolution of
protoplanetary disks, with implications for the composition of comets and
planets. This is the first of a series of papers based on data from DISCS, a
Submillimeter Array survey of the chemical composition of protoplanetary disks.
The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau
and MWC 480) range in stellar spectral type from M1 to A4 and offer an
opportunity to test the effects of stellar luminosity on the disk chemistry.
The disks were observed in 10 different lines at ~3" resolution and an rms of
~100 mJy beam-1 at ~0.5 km s-1. The four brightest lines are CO 2-1, HCO+ 3-2,
CN 2_3-1_2 and HCN 3-2 and these are detected toward all sources (except for
HCN toward CQ Tau). The weaker lines of CN 2_2-1_1, DCO+ 3-2, N2H+ 3-2, H2CO
3_03-2_02 and 4_14-3_13 are detected toward two to three disks each, and DCN
3-2 only toward LkCa 15. CH3OH 4_21-3_12 and c-C3H2 are not detected. There is
no obvious difference between the T Tauri and Herbig Ae sources with regard to
CN and HCN intensities. In contrast, DCO+, DCN, N2H+ and H2CO are detected only
toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack
cold regions for long enough timescales to allow for efficient deuterium
chemistry, CO freeze-out, and grain chemistry.Comment: 29 pages, 4 figures, accepted for publication in Ap
The Spitzer c2d legacy results: Star-formation rates and efficiencies; evolution and lifetimes
The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star-formation rates and efficiencies, numbers and lifetimes for spectral energy distribution (SED) classes, and clustering properties. Current star-formation efficiencies range from 3% to 6%; if star formation continues at current rates for 10 Myr, efficiencies could reach 15-30%. Star-formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M ☉ of stars per Myr. The star-formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star formation is still slow compared to that expected in a free-fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M ☉ pc–3. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M ☉, given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T bol, the values for L bol are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects
PLATO camera ghosts: simulations and measurements on the engineering model (EM)
The PLAnetary Transits and Oscillations of stars mission (PLATO) is the M3 mission in ESA’s Cosmic Vision 2015-2025 Programme, see Rauer et al. (2014).1 The PLATO mission aims at detecting and characterizing extrasolar planetary systems, including terrestrial exoplanets around bright solar-type stars up to the habitable zone. To be able to perform the required high precision photometric monitoring of the large target stars sample, PLATO is based on a multi-telescope configuration consisting of 26 Cameras, so as to provide simultaneously a large field of view and a large collecting aperture. The optical design is identical for all cameras and consists of a 6-lens dioptric design with a 120 mm entrance pupil and an effective field of view of more than 1000 square degrees. As for every optical system, especially dioptric ones, the presence of optical ghosts can dramatically affect the scientific observations. Thanks to the application of an excellent anti-reflection coating, PLATO’s cameras are by design very insensitive to ghosts. However, the residual faint back reflections focused on the detectors have to be simulated and considered during science operation (target selection) and in data correction algorithms. This article describes the different optical analyses performed to estimate the importance of ghosts in PLATO’s cameras, as well as the simulations performed to support the preparation of the test campaign on the first PLATO camera: the engineering model. Finally, the test execution, data analysis and results are presented and compared to the simulated data