Cloud L1642 Research Articles

Velocity gradient and stellar polarization: magnetic field tomography towards the L1688 cloud

ABSTRACT Magnetic fields are a defining yet enigmatic aspect of the interstellar medium, with their three-dimensional (3D) mapping posing a substantial challenge. In this study, we harness the innovative velocity gradient technique (VGT), underpinned by magnetohydrodynamic turbulence theories, to map the magnetic field structure by applying it to the atomic neutral hydrogen (H i) emission line and the molecular tracer 12CO. We construct the tomography of the magnetic field in the low-mass star-forming region L1688, utilizing two approaches: (1) VGT-H i combined with the Galactic rotational curve, and (2) stellar polarization paired with precise star parallax measurements. Our analysis reveals that the magnetic field orientations deduced from stellar polarization undergo a distinct directional change in the vicinity of L1688, providing evidence that the misalignment between VGT-H i and stellar polarization stems from the influence of the molecular cloud’s magnetic field on the polarization of starlight. When comparing VGT-12CO to stellar polarization and Planck polarization data, we observe that VGT-12CO effectively reconciles the misalignment noted with VGT-H i, showing statistical alignment with Planck polarization measurements. This indicates that VGT-12CO could be integrated with VGT-H i, offering vital insights into the magnetic fields of molecular clouds, thereby enhancing the accuracy of our 3D magnetic field reconstructions.

Revisiting the Magnetic Field of the L183 Starless Core

Abstract We present observations of linear polarization from dust thermal emission at 850 μm toward the starless cloud L183. These data were obtained at the James Clerk Maxwell Telescope (JCMT) using the SCUBA-2 camera in conjunction with its polarimeter POL-2. Polarized dust emission traces the plane-of-sky magnetic field structure in the cloud, thus allowing us to investigate the role of magnetic fields in the formation and evolution of its starless core. To interpret these measurements, we first calculate the dust temperature and column density in L183 by fitting the spectral energy distribution obtained by combining data from the JCMT and the Herschel space observatory. We used the Davis–Chandrasekhar–Fermi technique to measure the magnetic field strength in five subregions of the cloud, and we find values ranging from ∼120 ± 18 μG to ∼270 ± 64 μG in agreement with previous studies. Combined with an average hydrogen column density ( ) of ∼1.5 × 1022 cm−2 in the cloud, we also find that all five subregions are magnetically subcritical. These results indicate that the magnetic field in L183 is sufficiently strong to oppose the gravitational collapse of the cloud.

Segregation effect and N2 binding energy reduction in CO-N2 systems adsorbed on water ice substrates

Context. CO and N2 are two abundant species in molecular clouds. CO molecules are heavily depleted from the gas phase towards the centre of pre-stellar cores, whereas N2 maintains a high gas phase abundance. For example, in the molecular cloud L183, CO is depleted by a factor of ≈400 in its centre with respect to the outer regions of the cloud, whereas N2 is only depleted by a factor of ≈20. The reason for this difference is not yet clear, since CO and N2 have identical masses, similar sticking properties, and a relatively close energy of adsorption. Aims. We present a study of the CO-N2 system in sub-monolayer regimes, with the aim to measure, analyse and elucidate how the adsorption energy of the two species varies with coverage, with much attention to the case where CO is more abundant than N2. Methods. Experiments were carried out using the ultra-high vacuum (UHV) set-up called VENUS. Sub-monolayers of either pure 13CO or pure 15N2 and 13CO:15N2 mixtures were deposited on compact amorphous solid water ice, and crystalline water ice. Temperature-programmed desorption experiments, monitored by mass spectrometry, are used to analyse the distributions of binding energies of 13CO and 15N2 when adsorbed together in different proportions. Results. The distribution of binding energies of pure species varies from 990 K to 1630 K for 13CO, and from 890 K to 1430 K for 15N2. When a CO:N2 mixture is deposited, the 15N2 binding energy distribution is strongly affected by the presence of 13CO, whereas the adsorption energy of CO is unaltered. Conclusions. Whatever types of water ice substrate we used, the N2 effective binding energy was significantly lowered by the presence of CO molecules. We discuss the possible impact of this finding in the context of pre-stellar cores.

Matching dust emission structures and magnetic field in high-latitude cloud L1642: comparingHerschelandPlanckmaps

The nearby cloud L1642 is one of only two known very high latitude (|b| > 30 deg) clouds actively forming stars. It is a rare example of star formation in isolated conditions, and can reveal important details of star formation in general, e.g., of the effect of magnetic fields. We compare Herschel dust emission structures and magnetic field orientation revealed by Planck polarization maps in L1642. The high-resolution ($\sim20"$) Herschel data reveal a complex structure including a dense, compressed central clump, and low density striations. The Planck polarization data (at 10$'$ resolution) reveal an ordered magnetic field pervading the cloud and aligned with the surrounding striations. There is a complex interplay between the cloud structure and large scale magnetic field. This suggests that the magnetic field is closely linked to the formation and evolution of the cloud. CO rotational emission confirms that the striations are connected with the main clumps and likely to contain material either falling into or flowing out of the clumps. There is a clear transition from aligned to perpendicular structures approximately at a column density of $N_{\rm{H}} = 1.6 \times 10^{21}\, {{\rm cm}}^{-2}$. Comparing the Herschel maps with the Planck polarization maps shows the close connection between the magnetic field and cloud structure even in the finest details of the cloud.

On the importance of scattering at 8 μm: Brighter than you think

Context. Extinction and emission of dust models need for observational constraints to be validated. The coreshine phenomenon has already shown the importance of scattering in the 3 to 5 micron range and its ability to validate dust properties for dense cores. Aims. We want to investigate whether scattering can also play a role at longer wavelengths and to place even tighter constraints on the dust properties. Methods. We analyze the inversion of the Spitzer 8 micron map of the dense molecular cloud L183, to examine the importance of scattering as a potential contributor to the line-of-sight extinction. Results. The column density deduced from the inversion of the 8 micron map, when we neglect scattering, disagrees with all the other column density measurements of the same region. Modeling confirms that scattering at 8 microns is not negligible with an intensity of several hundred kJy per sr. This demonstrates the need of efficiently scattering dust grains at MIR wavelengths up to 8 microns. Coagulated aggregates are good candidates and might also explain the discrepancy at high extinction between E(J-K) et tau(9.7) toward dense molecular clouds. Further investigation requires considering efficiently scattering dust grains including ices as realistic dust models.

The VISTA Orion mini-survey: star formation in the Lynds 1630 North cloud

The Orion cloud complex presents a variety of star formation mechanisms and properties and it is still one of the most intriguing targets for star formation studies. We present VISTA/VIRCAM near-infrared observations of the L1630N star forming region, including the stellar clusters NGC 2068 and NGC 2071, in the Orion molecular cloud B and discuss them in combination with Spitzer data. We select 186 young stellar object (YSO) candidates in the region on the basis of multi-colour criteria, confirm the YSO nature of the majority of them using published spectroscopy from the literature, and use this sample to investigate the overall star formation properties in L1630N. The K-band luminosity function of L1630N is remarkably similar to that of the Trapezium cluster, i.e., it presents a broad peak in the range 0.3-0.7 M$_\odot$ and a fraction of sub-stellar objects of $\sim$20%. The fraction of YSOs still surrounded by disk/envelopes is very high ($\sim$85%) compared to other star forming regions of similar age (1-2 Myr), but includes some uncertain corrections for diskless YSOs. Yet, a possibly high disk fraction together with the fact that 1/3 of the cloud mass has a gas surface density above the threshold for star formation ($\sim$129 M$_\odot$ pc$^{-2}$), points towards a still on-going star formation activity in L1630N. The star formation efficiency (SFE), star formation rate (SFR) and density of star formation of L1630N are within the ranges estimated for galactic star forming regions by the Spitzer "core to disk" and "Gould's Belt" surveys. However, the SFE and SFR are lower than the average value measured in the Orion A cloud and, in particular, lower than that in the southern regions of L1630. This might suggest different star formation mechanisms within the L1630 cloud complex.

THE HERBIG BE STAR V1818 ORI AND ITS ENVIRONMENT

The little-studied Herbig Be star V1818 Ori is located in the direction of the southern L1641 cloud and the Mon R2 star-forming complex, and is most likely associated with the latter at a distance of ~900 pc. A high-resolution spectrum is consistent with a spectral type around B7 V, with lines of Hα, the red Ca ii triplet, and several forbidden lines in emission. An All Sky Automated Survey V-band light curve spanning 9 yr reveals major variability with deep absorption episodes reminiscent of the UX Orionis stars. We have searched for additional young stars clustering around V1818 Ori using grism images and the 2MASS and Wide-field Infrared Survey Explorer catalogs, and have found almost two dozen fainter stars with evidence of youth. Direct images show that the bright star IRAS 05510–1025, only about 3 arcmin from V1818 Ori, is surrounded by a reflection nebula, indicating its association with a molecular cloud. A spectrum of the star shows no emission-lines, and it is found to be a close binary with late B and early G type components. Its radial velocity indicates that it is an interloper, accidentally passing through the cloud and not physically associated with V1818 Ori.

Multiwavelength study of the high-latitude cloud L1642: chain of star formation

L1642 is one of the two high galactic latitude (|b| > 30deg) clouds confirmed to have active star formation. We examine the properties of this cloud, especially the large-scale structure, dust properties, and compact sources in different stages of star formation. We present high-resolution far-infrared and submm observations with the Herschel and AKARI satellites and mm observations with the AzTEC/ASTE telescope, which we combined with archive data from near- and mid-infrared (2MASS, WISE) to mm observations (Planck). The Herschel observations, combined with other data, show a sequence of objects from a cold clump to young stellar objects at different evolutionary stages. Source B-3 (2MASS J04351455-1414468) appears to be a YSO forming inside the L1642 cloud, instead of a foreground brown dwarf, as previously classified. Herschel data reveal striation in the diffuse dust emission around L1642. The western region shows striation towards NE and has a steeper column density gradient on its southern side. The densest central region has a bow-shock like structure showing compression from the west and a filamentary tail extending towards east. The differences suggest that these may be spatially distinct structures, aligned only in projection. We derive values of the dust emission cross-section per H nucleon for different regions of the cloud. Modified black-body fits to the spectral energy distribution of Herschel and Planck data give emissivity spectral index beta values 1.8-2.0 for the different regions. The compact sources have lower beta values and show an anticorrelation between T and beta. Markov chain Monte Carlo calculations demonstrate the strong anticorrelation between beta and T errors and the importance of mm Planck data in constraining the estimates. L1642 reveals a more complex structure and sequence of star formation than previously known.

HH 222: A GIANT HERBIG-HARO FLOW FROM THE QUADRUPLE SYSTEM V380 ORI

HH 222 is a giant shocked region in L1641 cloud, and is popularly known as Orion Streamers or the waterfall on account of its unusual structure. At center of these streamers are two infrared sources coincident with a nonthermal radio jet aligned along principal streamer. The unique morphology of HH 222 has long been associated with this radio jet. However, new infrared images show that two sources are distant elliptical galaxies, indicating that radio jet is merely an improbable line-of-sight coincidence. Accurate proper motion measurements of HH 222 reveal that shock structure is a giant bow shock moving directly away from well-known, very young, Herbig Be star V380 Ori. The already known Herbig-Haro object HH 35 forms part of this flow. A new Herbig-Haro object, HH 1041, is found precisely in opposite direction of HH 222 and is likely to form part of a counterflow. The total projected extent of this HH complex is 5.3 pc, making it among largest HH flows known. A second outflow episode from V380 Ori is identified as a pair of HH objects, HH 1031 to northwest and already known HH 130 to southeast, along an axis that deviates from that of HH 222/HH 1041 by only 37. V380 Ori is a hierarchical quadruple system, including a faint companion of spectral type M5 or M6, which at an age of ~1 Myr corresponds to an object straddling stellar-to-brown dwarf boundary. We suggest that HH 222 giant bow shock is a direct result of dynamical interactions that led to conversion from an initial non-hierarchical multiple system into a hierarchical configuration. This event occurred no more than 28,000 yr ago, as derived from proper motions of HH 222 giant bow shock.

Molecular outflows identified in the FCRAO CO survey of the Taurus Molecular Cloud

Jets and outflows are an integral part of the star formation process. While there are many detailed studies of molecular outflows towards individual star-forming sites, few studies have surveyed an entire star-forming molecular cloud for this phenomenon. The 100-deg2 Five College Radio Astronomy Observatory CO survey of the Taurus Molecular Cloud provides an excellent opportunity to undertake an unbiased survey of a large, nearby, molecular cloud complex for molecular outflow activity. Our study provides information on the extent, energetics and frequency of outflows in this region, which are then used to assess the impact of outflows on the parent molecular cloud. The search identified 20 outflows in the Taurus region, eight of which were previously unknown. Both 12CO and 13CO data cubes from the Taurus molecular map were used, and dynamical properties of the outflows are derived. Even for previously known outflows, our large-scale maps indicate that many of the outflows are much larger than previously suspected, with eight of the outflows (40 per cent) being more than a parsec long. The mass, momentum and kinetic energy from the 20 outflows are compared to the repository of turbulent energy in Taurus. Comparing the energy deposition rate from outflows to the dissipation rate of turbulence, we conclude that outflows by themselves cannot sustain the observed turbulence seen in the entire cloud. However, when the impact of outflows is studied in selected regions of Taurus, it is seen that locally outflows can provide a significant source of turbulence and feedback. The L1551 dark cloud which is just south of the main Taurus complex was not covered by this survey, but the outflows in L1551 have much higher energies compared to the outflows in the main Taurus cloud. In the L1551 cloud, outflows can not only account for the turbulent energy present, but are probably also disrupting their parent cloud. We conclude that for a molecular cloud like Taurus, an L1551-like episode occurring once every 105 years is sufficient to sustain the turbulence observed. Five of the eight newly discovered outflows have no known associated stellar source, indicating that they may be embedded Class 0 sources. In Taurus, 30 per cent of Class I sources and 12 per cent of flat-spectrum sources from the Spitzer young stellar object (YSO) catalogue have outflows, while 75 per cent of known Class 0 objects have outflows. Overall, the paucity of outflows in Taurus compared to the embedded population of Class I and flat-spectrum YSOs indicates that molecular outflows are a short-lived stage marking the youngest phase of protostellar life. The current generation of outflows in Taurus highlight an ongoing period of active star formation, while a large fraction of YSOs in Taurus have evolved well past the Class I stage.

Extended emission of D2H+in a prestellar core

Context: In the last years, the H2D+ and D2H+ molecules have gained great attention as probes of cold and depleted dense molecular cloud cores. These ions are at the basis of molecular deuterium fractionation, a common characteristic observed in star forming regions. H2D+ is now routinely observed, but the search for its isotopologue D2H+ is still difficult because of the high frequency of its ground para transition (692 GHz). Aims: We have observed molecular transitions of H2D+ and D2H+ in a cold prestellar core to characterize the roots of deuterium chemistry. Methods: Thanks to the sensitive multi-pixel CHAMP+ receiver on the APEX telescope where the required excellent weather conditions are met, we not only successfully detect D2H+ in the H-MM1 prestellar core located in the L1688 cloud, but also obtain information on the spatial extent of its emission. We also detect H2D+ at 372 GHz in the same source. We analyse these detections using a non-LTE radiative transfer code and a state-of-the-art spin-dependent chemical model. Results: This observation is the first secure detection of D2H+ in space. The emission is moreover extended over several pixels of the CHAMP+ array, i.e. on a scale of at least 40'', corresponding to ~ 4800 AU. We derive column densities on the order of 1e12-1e13 cm-2 for both molecules in the LTE approximation depending on the assumed temperature, and up to two orders of magnitude higher based on a non-LTE analysis. Conclusions: Our modeling suggests that the level of CO depletion must be extremely high (>10, and even >100 if the temperature of the core is around 10 K) at the core center, in contradiction with CO depletion levels directly measured in other cores. Observation of the H2D+ spatial distribution and direct measurement of the CO depletion in H-MM1 will be essential to confirm if present chemical models investigating the basis of deuterium [...].

FAINT COLLIMATED HERBIG-HARO JETS FROM VISIBLE STARS IN L1641

A population of 11 faint, collimated jets has been discovered in the northern part of the L1641 cloud in the region of HH 1/2, HH 34, and the L1641-N cluster. These jets were missed in previous imaging surveys on account of their weak emission, and they were discovered only on deep exposures with the Subaru 8 m telescope. With these new faint jets, the number of HH flows within the area surveyed has doubled. This suggests that collimated jets from young stars may be more common than previously assumed. It is noteworthy that all of the jets are associated with optically visible stars with r magnitudes ranging from 13.8 to 22.0. The driving sources of jets in regions flooded by ultraviolet radiation from nearby OB stars are known to be excavated by photoionization, and in three cases remnant Hα emission envelopes are found associated with the sources, although the more benign environment in the region observed here, about 10 pc distant from the Orion Nebula Cluster, makes the optical visibility of all these sources rather surprising. Such faint jets from visible stars represent either the final vestiges of the outflow phenomenon, or they are triggered by disturbances of the remnant disks, possibly initiated by the orbital evolution of binaries that spiral in to form close binaries. Among the known Hα emission stars within the region surveyed, 8% are found to be associated with jets.

HH 212: Submillimeter Array Observations of a Remarkable Protostellar Jet

HH 212 is a nearby (460 pc) protostellar jet discovered in H2 powered by a Class 0 source, IRAS 05413-0104, in the L1630 cloud of Orion. It is highly collimated and symmetric with matched pairs of bow shocks on either side of the source. We have mapped it in 850 µm continuum, SiO (J = 8 7), CO (J = 3 2), SO (NJ = 89 78), HCO + (J = 4 3), and H 13 CO + (J = 4 3) emission simultaneously at � 1 ′′ resolution with the Submillimeter Array (SMA). Thermal dust emission is seen in continuum around the source, mainly arising from an inner envelope (i.e., the inner part of a previously seen flattened envelope) and a possible disk. The inner envelope is also seen with rotation in CO, HCO + , and probably SO. Like H2 emission, CO and SiO emission are seen along the jet axis but extending closer to the source, tracing the bow shocks with a broad range of velocities and the continuous structures in between. SO emission is seen only around the source, forming a jetlike structure extending along the jet axis from the source, likely tracing the jet near the launching region. The jet is episodic and bending. It may also be slightly precessing as the jetlike SO structure shows a slight S-shaped symmetry about the source. A hint of jet rotation is also seen across the jet axis. Internal outflow shells are seen in CO and HCO + , associated with the bow shocks in the inner part of the jet. The bases of the HCO + shells are seen with a hint of rotation similar to that seen in the inner envelope, probably consisted mainly of the material extended from the inner envelope and even the possible disk. The bases of the outflow shells are also seen in H 13 CO + and even the continuum, probably tracing the dense material extended from around the same regions. Outflow shells are also seen in CO surrounding the jet extending out from the H2 nebulae seen around the source. Subject headings: stars: formation — ISM: individual: HH 212 — ISM: jets and outflows.

Organic Chemistry in the Dark Clouds L1448 and L183: A Unique Grain Mantle Composition

We present high sensitivity observations of the complex organic molecules (COMs) CH3OH, C2H5OH, HCOOCH3, HCOOH and H2CO and of SiO toward the quiescent dark cloud L183 and the molecular outflow L1448-mm. We have not detected C2H5OH, HCOOCH3 and SiO in L183 and in the quiescent gas of L1448-mm. The abundances of CH3OH, H2CO and SiO are enhanced by factors 4-20 in the shock precursor component, and those of CH3OH and SiO by 3 and 4 orders of magnitude in the shocked gas, without substantial changes (< factor of 2) in the abundances of C2H5OH and HCOOCH3 relative to that of CH3OH. The large enhancements of SiO and CH3OH can be explained by the shock ejection of an important fraction of the grain mantle material into gas phase. Our upper limits to the C2H5OH/CH3OH and HCOOCH3/CH3OH ratios are consistent with the rather constant ratios measured in hot cores and Galactic center clouds. However, the upper limits to the HCOOCH3/CH3OH and the HCOOH/CH3OH ratios are at least one order of magnitude smaller than those found in"hot corinos" surrounding low mass protostars. We speculate that the observed abundances of COMs in different objects are consistent with a sort of "universal" grain mantle composition which is locally changed by the processes of low mass star formation.

Entrainment Mechanisms for Outflows in the L1551 Star‐forming Region

We present high sensitivity 12/13CO(1-0) molecular line maps covering the full extent of the parsec scale L1551 molecular outflow, including the redshifted east-west (EW) flow. We also present 12CO(3-2) data that extends over a good fraction of the area mapped in the 1-0 transition. We compare the molecular data to widefield, narrow-band optical emission in H$\alpha$. While there are multiple outflows in the L1551 cloud, the main outflow is oriented at 50\arcdeg position angle and appears to be driven by embedded source(s) in the central IRS 5 region. The 3-2 data indicate that there may be molecular emission associated with the L1551 NE jet, within the redshifted lobe of main outflow. We have also better defined the previously known EW flow and believe we have identified its blueshifted counterpart. We further speculate that the origin of the EW outflow lies near HH 102. We use velocity dependent opacity correction to estimate the mass and the energy of the outflow. The resulting mass spectral indices from our analysis, are systematically lower (less steep) than the power law indices obtained towards other outflows in several recent studies that use a similar opacity correction method. We show that systematic errors and biases in the analysis procedures for deriving mass spectra could result in errors in the determination of the power-law indices. The mass spectral indices, the morphological appearance of the position-velocity plots and integrated intensity emission maps of the molecular data, compared with the optical, suggest that jet-driven bow-shock entrainment is the best explanation for the driving mechanism of outflows in L1551. The kinetic energy of the outflows is found to be comparable to the binding energy of the cloud and sufficient to maintain the turbulence in the L1551 cloud.

V1647 Orionis: The X-Ray Evolution of a Pre-Main-Sequence Accretion Burst

We present Chandra X-ray Observatory monitoring observations of the recent accretion outburst displayed by the pre-main sequence (pre-MS) star V1647 Ori. The X-ray observations were obtained over a period beginning prior to outburst onset in late 2003 and continuing through its apparent cessation in late 2005, and demonstrate that the mean flux of the spatially coincident X-ray source closely tracked the near-infrared luminosity of V1647 Ori throughout its eruption. We find negligible likelihood that the correspondence between X-ray and infrared light curves over this period was the result of multiple X-ray flares unrelated to the accretion burst. The recent Chandra data confirm that the X-ray spectrum of V1647 Ori hardened during outburst, relative both to its pre-outburst state and to the X-ray spectra of nearby pre-MS stars in the L1630 cloud. We conclude that the observed changes in the X-ray emission from V1647 Ori over the course of its 2003-2005 eruption were generated by a sudden increase and subsequent decline in its accretion rate. These results for V1647 Ori indicate that the flux of hard X-ray emission from erupting low-mass, pre-MS stars, and the duration and intensity of such eruptions, reflect the degree to which star-disk magnetic fields are reorganized before and during major accretion events.

Detection of H2D$\mathsf{^+}$ in a massive prestellar core in Orion B

Aims. The purpose of this study is to examine the prediction that the deuterated H + 3 ion, H 2 D + , can be found exclusively in the coldest regions of molecular cloud cores. This is also a feasibility study for the detection of the ground-state line of ortho-H 2 D + at 372 GHz with APEX. Methods. The (1 10 → 1 11 ) transition of H 2 D + at 372 GHz was searched towards selected positions in the massive star forming cloud OriB9, in the dark cloud L183, and in the low-to intermediate mass star-forming cloud R CrA. Results. The line was detected in cold, prestellar cores in the regions of OriB9 and L183, but only upper limits were obtained towards other locations which either have elevated temperatures or contain a newly born star. The H 2 D + detection towards OriB9 is the first one in a massive star-forming region. The fractional ortho-H 2 D + abundances (relative to H 2 ) are estimated to be ∼ 1 × 10 -10 in two cold cores in OriB9, and 3 x 10 -10 in the cold core of L183. Conclusions. The H 2 D + detection in OriB9 shows that also massive star forming regions contain very cold prestellar cores which probably have reached matured chemical composition characterized, e.g., by a high degree of deuterium fractionation. Besides as a tracer of the interior parts of prestellar cores, H 2 D + may therefore be used to put contraints on the timescales related to massive star formation.

A SCUBA survey of L1689 - the dog that didn't bark

We present submillimetre data for the L1689 cloud in the ρ Ophiuchi molecular cloud complex. We detect a number of starless and pre-stellar cores and protostellar envelopes. We also detect a number of filaments for the first time in the submillimetre continuum that are parallel both to each other, and to filaments observed in the neighbouring L1688 cloud. These filaments are also seen in the 13 CO observations of L1689. The filaments contain all of the star-formation activity in the cloud. L1689 lies next to the well-studied L1688 cloud that contains the ρ Oph-A core. L1688 has a much more active star-formation history than L1689 despite their apparent similarity in 13 CO data. Hence, we label L1689 as the dog that didn’t bark. We endeavour to explain this apparent anomaly by comparing the total mass of each cloud that is currently in the form of dense material such as pre-stellar cores. We note firstly that L1688 is more massive than L1689, but we also find that when normalized to the total mass of each cloud, the L1689 cloud has a much lower percentage of mass in dense cores than L1688. We attribute this to the hypothesis of Loren that the star formation in the ρ Ophiuchi complex is being affected and probably dominated by the external influence of the nearby Upper Scorpius OB association and predominantly by σ Sco. L1689 is further from σ Sco and is therefore less active. The influence of σ Sco appears none the less to have created the filaments that we observe in L1689.

Infall and Outflow around the HH 212 Protostellar System

HH 212 is a highly collimated jet discovered in H2 powered by a young Class 0 source, IRAS 05413-0104, in the L1630 cloud of Orion. We have mapped around it in 1.33 mm continuum, 12CO ($J=2-1$), 13CO ($J=2-1$), C18O ($J=2-1$), and SO ($J_K = 6_5-5_4$) emission at $\sim$ \arcs{2.5} resolution with the Submillimeter Array. A dust core is seen in the continuum around the source. A flattened envelope is seen in C18O around the source in the equator perpendicular to the jet axis, with its inner part seen in 13CO. The structure and kinematics of the envelope can be roughly reproduced by a simple edge-on disk model with both infall and rotation. In this model, the density of the disk is assumed to have a power-law index of $p=-1.5$ or -2, as found in other low-mass envelopes. The envelope seems dynamically infalling toward the source with slow rotation because the kinematics is found to be roughly consistent with a free fall toward the source plus a rotation of a constant specific angular momentum. A 12CO outflow is seen surrounding the H2 jet, with a narrow waist around the source. Jetlike structures are also seen in 12CO near the source aligned with the H2 jet at high velocities. The morphological relationship between the H2 jet and the 12CO outflow, and the kinematics of the 12CO outflow along the jet axis are both consistent with those seen in a jet-driven bow shock model. SO emission is seen around the source and the H2 knotty shocks in the south, tracing shocked emission around them.

Multigenerational Star Formation in L1551

The L1551 molecular cloud contains two small clusters of Class 0 and I protostars, as well as a halo of more evolved Class II and III YSOs, indicating a current and at least one past burst of star formation. We present here new, sensitive maps of 850 and 450 um dust emission covering most of the L1551 cloud, new CO J=2-1 data of the molecular cloud, and a new, deep, optical image of [SII] emission. No new Class 0/I YSOs were detected. Compact sub-millimetre emitters are concentrated in two sub-clusters: IRS5 and L1551NE, and the HL~Tauri group. Both stellar groups show significant extended emission and outflow/jet activity. A jet, terminating at HH 265 and with a very weak associated molecular outflow, may originate from LkHa 358, or from a binary companion to another member of the HL Tauri group. Several Herbig Haro objects associated with IRS5/NE were clearly detected in the sub-mm, as were faint ridges of emission tracing outflow cavity walls. We confirm a large-scale molecular outflow originating from NE parallel to that from IRS5, and suggest that the hollow shell morphology is more likely due to two interacting outflows. We confirm the presence of a prestellar core (L1551-MC) of mass 2-3 Mo north-west of IRS5. The next generation cluster may be forming in this core. The L1551 cloud appears cometary in morphology, and appears to be illuminated and eroded from the direction of Orion, perhaps explaining the multiple episodes of star formation in this cloud. The full paper (including figures) can be downloaded at this http URL, or viewed at this http URL