Intermediate-mass Protostar Research Articles

Episodic excursions of low-mass protostars on the Hertzsprung–Russell diagram

Following our recent work devoted to the effect of accretion on the pre-main-sequence evolution of low-mass stars, we perform a detailed analysis of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R) diagram triggered by strong mass accretion bursts typical of FU Orionis-type objects (FUors). These excursions reveal themselves as sharp increases in the stellar total luminosity and/or effective temperature of the protostar and can last from hundreds to a few thousands of years, depending on the burst strength and characteristics of the protostar. During the excursions, low-mass protostars occupy the same part of the H-R diagram as young intermediate-mass protostars in the quiescent phase of accretion. Moreover, the time spent by low-mass protostars in these regions is on average a factor of several longer than that spent by the intermediate-mass stars in quiescence. During the excursions, low-mass protostars pass close to the position of most known FUors in the H-R diagram, but owing to intrinsic ambiguity the model stellar evolutionary tracks are unreliable in determining the FUor properties. We find that the photospheric luminosity in the outburst state may dominate the accretion luminosity already after a few years after the onset of the outburst, meaning that the mass accretion rates of known FUors inferred from the bolometric luminosity may be systematically overestimated, especially in the fading phase.

First hot corino detected around an isolated intermediate-mass protostar: Cep E-mm

Context. Intermediate-mass (IM) protostars provide a bridge between the low- and high-mass protostars. Despite their relevance, little is known about their chemical diversity. Aims. We want to investigate the molecular richness towards the envelope of I-M protostars and to compare their properties with those of low- and high-mass sources. Methods. We have selected the isolated IM Class 0 protostar Cep E-mm to carry out an unbiased molecular survey with the IRAM 30 m telescope between 72 and 350 GHz with an angular resolution lying in the range 7–34″. Our goal is to obtain a census of the chemical content of the protostellar envelope. These data were complemented with NOEMA observations of the spectral bands 85.9–89.6 GHz and 216.8–220.4 GHz at angular resolutions of 2.3″ and 1.4″, respectively. Results. The 30 m spectra show bright emission of O- and N-bearing complex organic molecules (COMs): CH3OH and its rare isotopologues CH2DOH and 13CH3OH, CH3CHO, CH3OCH3, CH3COCH3, HCOOH, HCOOCH3, H2CCO, NH2CHO, CH3CN, C2H3CN, C2H5CN, HNCO and H2CO. We identify up to three components in the spectral signature of COMs: an extremely broad line (eBL) component associated with the outflowing gas (FWHM > 7kms−1), a narrow line (NL) component (FWHM < 3kms−1) associated with the cold envelope, and a broad line (BL) component (FWHM ≃ 5.5kms−1) which traces the signature of a hot corino. The eBL and NL components are detected only in molecular transitions of low excitation and dominate the emission of CH3OH. The BL component is detected in highly excited gas (Eup > 100 K). The NOEMA observations reveal Cep E-mm as a binary protostellar system, whose components, Cep E-A and Cep E-B, are separated by ≈1.7″. Cep E-A dominates the core continuum emission and powers the long-studied, well-known, high-velocity jet associated with HH377. The lower flux source Cep E-B powers another high-velocity molecular jet, reaching velocities of ≈80 km s−1, which propagates in a direction close to perpendicular with respect to the Cep E-A jet. Our interferometric maps show that the emission of COMs arises from a region of ≈0.7″ size around Cep E-A, and corresponds to the BL component detected with the IRAM 30 m telescope. On the contrary, no COM emission is detected towards Cep E-B. We have determined the rotational temperature (Trot) and the molecular gas column densities from a simple population diagram analysis or assuming a given excitation temperature. Rotational temperatures of COMs emission were found to lie in the range 20−40 K with column densities ranging from a few times 1015 cm−2 for O-bearing species, down to a few times 1014 cm−2 for N-bearing species. Molecular abundances are similar to those measured towards other low- and intermediate-mass protostars. Ketene (H2CCO) appears as an exception, as it is found significantly more abundant towards Cep E-A. High-mass hot cores are significantly less abundant in methanol and N-bearing species are more abundant by two to three orders of magnitude. Conclusions. Cep E-mm reveals itself as a binary protostellar system with a strong chemical differentiation between both cores. Only the brightest component of the binary is associated with a hot corino. Its properties are similar to those of low-mass hot corinos.

Proper motions of collimated jets from intermediate-mass protostars in the Carina Nebula

We present proper motion measurements of 37 jets and HH objects in the Carina Nebula measured in two epochs of H$\alpha$ images obtained $\sim 10$ yrs apart with HST/ACS. Transverse velocities in all but one jet are faster than $\gtrsim 25$ km s$^{-1}$, confirming that the jet-like H$\alpha$ features identified by Smith et al. (2010) trace outflowing gas. Proper motions constrain the location of the jet-driving source and provide kinematic confirmation of the intermediate-mass protostars that we identify for 20/37 jets. Jet velocities do not correlate with the estimated protostar mass and embedded driving sources do not have slower jets. Instead, transverse velocities (median $\sim 75$ km s$^{-1}$) are similar to those in jets from low-mass stars. Assuming a constant velocity since launch, we compute jet dynamical ages (median $\sim 10^4$ yr). If continuous emission from inner jets traces the duration of the most recent accretion bursts, then these episodes are sustained longer (median $\sim 700$ yr) than the typical decay time of an FU Orionis outburst. These jets can carry appreciable momentum that may be injected into the surrounding environment. The resulting outflow force, $dP/dt$, lies between that measured in low- and high-mass sources, despite the very different observational tracers used. Smooth scaling of the outflow force argues for a common physical process underlying outflows from protostars of all masses. This latest kinematic result adds to a growing body of evidence that intermediate-mass star formation proceeds like a scaled-up version of the formation of low-mass stars.

Star Formation Under the Outflow: The Discovery of a Non-thermal Jet from OMC-2 FIR 3 and Its Relationship to the Deeply Embedded FIR 4 Protostar

Abstract We carried out multiwavelength (0.7–5 cm), multi-epoch (1994–2015) Very Large Array (VLA) observations toward the region enclosing the bright far-IR sources FIR 3 (HOPS 370) and FIR 4 (HOPS 108) in OMC-2. We report the detection of 10 radio sources, 7 of them identified as young stellar objects. We image a well-collimated radio jet with a thermal free–free core (VLA 11) associated with the Class I intermediate-mass protostar HOPS 370. The jet features several knots (VLA 12N, 12C, 12S) of non-thermal radio emission (likely synchrotron from shock-accelerated relativistic electrons) at distances of ∼7500–12,500 au from the protostar, in a region where other shock tracers have been previously identified. These knots are moving away from the HOPS 370 protostar at ∼100 km s−1. The Class 0 protostar HOPS 108, which itself is detected as an independent, kinematically decoupled radio source, falls in the path of these non-thermal radio knots. These results favor the previously proposed scenario in which the formation of HOPS 108 is triggered by the impact of the HOPS 370 outflow with a dense clump. However, HOPS 108 has a large proper motion velocity of ∼30 km s−1, similar to that of other runaway stars in Orion, whose origin would be puzzling within this scenario. Alternatively, an apparent proper motion could result because of changes in the position of the centroid of the source due to blending with nearby extended emission, variations in the source shape, and/or opacity effects.

Dust and gas environment of the young embedded cluster IRAS 18511+0146

IRAS 18511+0146 is a young embedded (proto)cluster located at 3.5 kpc surrounding what appears to be an intermediate mass protostar. In this paper, we investigate the nature of cluster members (two of which are believed to be the most massive and luminous) using imaging and spectroscopy in the near and mid-infrared. The brightest point-like object associated with IRAS 18511+0146 is referred to as S7 in the present work (designated UGPS J185337.88+015030.5 in the UKIRT Galactic Plane survey). Seven of the nine objects show rising spectral energy distributions (SED) in the near-infrared, with four objects showing Br-gamma emission. Three members: S7, S10 (also UGPS J185338.37+015015.3) and S11 (also UGPS J185338.72+015013.5) are bright in mid-infrared with diffuse emission being detected in the vicinity of S11 in PAH bands. Silicate absorption is detected towards these three objects, with an absorption maximum between 9.6 and 9.7 um, large optical depths (1.8-3.2), and profile widths of 1.6-2.1 um. The silicate profiles of S7 and S10 are similar, in contrast to S11 (which has the largest width and optical depth). The cold dust emission investigated using Herschel HiGal peaks at S7, with temperature at 26 K and column density N(H2) ~ 7 x 10^(22) cm^(-2). The bolometric luminosity of IRAS 18511 region is L ~ 1.8 x 10^4 L_sun. S7 is the main contributor to the bolometric luminosity, with L (S7) > 10^4 L_sun. S7 is a high mass protostellar object with ionised stellar winds, evident from the correlation between radio and bolometric luminosity as well as the asymmetric Br-gamma profile. The differences in silicate profiles of S7 and S11 could be due to different radiation environment as we believe the former to be more massive and in an earlier phase than the latter.

Herschel/PACS far-IR spectral imaging of a jet from an intermediate mass protostar in the OMC-2 region

We present the first detection of a jet in the far-IR [O I] lines from an intermediate mass protostar. We have carried out a Herschel/PACS spectral mapping study in the [O I] lines of OMC-2 FIR 3 and FIR 4, two of the most luminous protostars in Orion outside of the Orion Nebula. The spatial morphology of the fine structure line emission reveals the presence of an extended photodissociation region (PDR) and a narrow, but intense jet connecting the two protostars. The jet seen in [O I] emission is spatially aligned with the Spitzer/IRAC 4.5 micron jet and the CO (6-5) molecular outflow centered on FIR 3. The mass loss rate derived from the total [O I] 63 micron line luminosity of the jet is 7.7 x 10^-6 M_sun/yr, more than an order of magnitude higher than that measured for typical low mass class 0 protostars. The implied accretion luminosity is significantly higher than the observed bolometric luminosity of FIR 4, indicating that the [O I] jet is unlikely to be associated with FIR 4. We argue that the peak line emission seen toward FIR 4 originates in the terminal shock produced by the jet driven by FIR 3. The higher mass-loss rate that we find for FIR 3 is consistent with the idea that intermediate mass protostars drive more powerful jets than their low-mass counterparts. Our results also call into question the nature of FIR 4.

Fe ii] jets from intermediate-mass protostars in Carina

We present new HST/WFC3-IR narrowband [Fe II] images of protostellar jets in the Carina Nebula. Combined with 5 previously published sources, we have a sample of 18 jets and 2 HH objects. All of the jets we targeted with WFC3 show bright infrared [Fe II] emission, and a few H$\alpha$ candidate jets are confirmed as collimated outflows based on the morphology of their [Fe II] emission. Continuum-subtracted images clearly separate jet emission from the adjacent ionization front, providing a better tracer of the collimated jet than H$\alpha$ and allowing us to connect these jets with their embedded driving sources. The [Fe II] 1.64 $\mu$m/H$\alpha$ flux ratio measured in the jets is $\gtrsim 5$ times larger than in the adjacent ionization fronts. The low-ionization jet core requires high densities to shield Fe$^+$ against further ionization by the FUV radiation from O-type stars in the H II region. High jet densities imply high mass-loss rates, consistent with the intermediate-mass driving sources we identify for 13 jets. The remaining jets emerge from opaque globules that obscure emission from the protostar. In many respects, the HH jets in Carina look like a scaled-up version of the jets driven by low-mass protostars. Altogether, these observations suggest that [Fe II] emission is a reliable tracer of dense, irradiated jets driven by intermediate-mass protostars. We argue that highly collimated outflows are common to more massive protostars, and that they suggest the outflow physics inferred for low-mass stars formation scales up to at least $\sim8$ M$_{\odot}$.

Connecting low- and high-mass star formation: the intermediate-mass protostar IRAS 05373+2349 VLA 2

Until recently, there have been few studies of the protostellar evolution of intermediate-mass (IM) stars, which may bridge the low- and high-mass regimes. This paper aims to investigate whether the properties of an IM protostar within the IRAS 05373+2349 embedded cluster are similar to that of low- and/or high-mass protostars. We carried out Very Large Array as well as Combined Array for Research in Millimeter Astronomy continuum and 12CO(J=1-0) observations, which uncover seven radio continuum sources (VLA 1-7). The spectral index of VLA 2, associated with the IM protostar is consistent with an ionised stellar wind or jet. The source VLA 3 is coincident with previously observed H2 emission line objects aligned in the north-south direction (P.A. -20 to -12 deg), which may be either an ionised jet emanating from VLA 2 or (shock-)ionised cavity walls in the large-scale outflow from VLA 2. The position angle between VLA 2 and 3 is slightly misaligned with the large-scale outflow we map at ~5-arcsec resolution in 12CO (P.A. ~30 deg), which in the case of a jet suggests precession. The emission from the mm core associated with VLA 2 is also detected; we estimate its mass to be 12-23 Msun, depending on the contribution from ionised gas. Furthermore, the large-scale outflow has properties intermediate between outflows from low- and high-mass young stars. Therefore, we conclude that the IM protostar within IRAS 05373+2349 is phenomenologically as well as quantitatively intermediate between the low- and high-mass domains.

Revealing the dynamics of Class 0 protostellar discs with ALMA

We present synthetic ALMA observations of Keplerian, protostellar discs in the Class 0 stage studying the emission of molecular tracers like $^{13}$CO, C$^{18}$O, HCO$^+$, H$^{13}$CO$^+$, N$_2$H$^+$, and H$_2$CO. We model the emission of discs around low- and intermediate-mass protostars. We show that under optimal observing conditions ALMA is able to detect the discs already in the earliest stage of protostellar evolution, although the emission is often concentrated to the innermost 50 AU. Therefore, a resolution of a few 0.1" might be too low to detect Keplerian discs around Class 0 objects. We also demonstrate that under optimal conditions for edge-on discs Keplerian rotation signatures are recognisable, from which protostellar masses can be inferred. For this we here introduce a new approach, which allows us to determine protostellar masses with higher fidelity than before. Furthermore, we show that it is possible to reveal Keplerian rotation even for strongly inclined discs and that ALMA should be able to detect possible signs of fragmentation in face-on discs. In order to give some guidance for future ALMA observations, we investigate the influence of varying observing conditions and source distances. We show that it is possible to probe Keplerian rotation in inclined discs with an observing time of 2 h and a resolution of 0.1", even in the case of moderate weather conditions. Furthermore, we demonstrate that under optimal conditions, Keplerian discs around intermediate-mass protostars should be detectable up to kpc-distances.

Outflow forces in intermediate-mass star formation

Context. Protostars of intermediate-mass provide a bridge between theories of low- and high-mass star formation. Molecular outflows emerging from such sources can be used to determine the influence of fragmentation and multiplicity on protostellar evolution through the apparent correlation of outflow forces of intermediate-mass protostars with the total luminosity instead of the individual luminosity.

INVESTIGATING PARTICLE ACCELERATION IN PROTOSTELLAR JETS: THE TRIPLE RADIO CONTINUUM SOURCE IN SERPENS

ABSTRACT While most protostellar jets present free–free emission at radio wavelengths, synchrotron emission has also been proposed to be present in a handful of these objects. The presence of nonthermal emission has been inferred by negative spectral indices at centimeter wavelengths. In one case (the HH 80-81 jet arising from a massive protostar), its synchrotron nature was confirmed by the detection of linearly polarized radio emission. One of the main consequences of these results is that synchrotron emission implies the presence of relativistic particles among the nonrelativistic material of these jets. Therefore, an acceleration mechanism should be taking place. The most probable scenario is that particles are accelerated when the jets strongly impact against the dense envelope surrounding the protostar. Here we present an analysis of radio observations obtained with the Very Large Array of the triple radio source in the Serpens star-forming region. This object is known to be a radio jet arising from an intermediate-mass protostar. It is also one of the first protostellar jets where the presence of nonthermal emission was proposed. We analyze the dynamics of the jet and the nature of the emission and discuss these issues in the context of the physical parameters of the jet and the particle acceleration phenomenon.

MILLIMETER SOURCE 13 S IN R CrA: OBSERVATIONS OF A PROTO-HERBIG Ae SYSTEM CANDIDATE

Compelled to observe a young, intermediate mass (IM) protostar candidate, we image velocity structure in a variety of molecular lines toward a strong millimeter continuum peak, MMS 13, in the young stellar object cluster R Corona Australis. We report the detection of N2H+ (1–0) and C34S (2–1) corresponding to a filamentary IM core coincident with a site of turbulent flow convergence and Spitzer infrared emission minimum. Several emission features including a central decay of turbulence, kinematic signatures of bulk infall, and strong winds support mass and age estimates in indicating the presence of a proto-Herbig Ae system in the region. Channeling of center velocity flow with turbulence amplification along the rotation axis drives wind generation in the system candidate.

Far-infrared CO and H2O emission in intermediate-mass protostars

Intermediate-mass young stellar objects (YSOs) provide a link to understand how feedback from shocks and UV radiation scales from low to high-mass star forming regions. Aims: Our aim is to analyze excitation of CO and H$_2$O in deeply-embedded intermediate-mass YSOs and compare with low-mass and high-mass YSOs. Methods: Herschel/PACS spectral maps are analyzed for 6 YSOs with bolometric luminosities of $L_\mathrm{bol}\sim10^2 - 10^3$ $L_\odot$. The maps cover spatial scales of $\sim 10^4$ AU in several CO and H$_2$O lines located in the $\sim55-210$ $\mu$m range. Results: Rotational diagrams of CO show two temperature components at $T_\mathrm{rot}\sim320$ K and $T_\mathrm{rot}\sim700-800$ K, comparable to low- and high-mass protostars probed at similar spatial scales. The diagrams for H$_2$O show a single component at $T_\mathrm{rot}\sim130$ K, as seen in low-mass protostars, and about $100$ K lower than in high-mass protostars. Since the uncertainties in $T_\mathrm{rot}$ are of the same order as the difference between the intermediate and high-mass protostars, we cannot conclude whether the change in rotational temperature occurs at a specific luminosity, or whether the change is more gradual from low- to high-mass YSOs. Conclusions: Molecular excitation in intermediate-mass protostars is comparable to the central $10^{3}$ AU of low-mass protostars and consistent within the uncertainties with the high-mass protostars probed at $3\cdot10^{3}$ AU scales, suggesting similar shock conditions in all those sources.

FRAGMENTATION OF MOLECULAR CLUMPS AND FORMATION OF A PROTOCLUSTER

Sufficiently massive clumps of molecular gas collapse under self-gravity and fragment to spawn a cluster of stars that have a range of masses. We investigate observationally the early stages of formation of a stellar cluster in a massive filamentary infrared dark cloud, G28.34+0.06 P1, in the 1.3mm continuum and spectral line emission using the ALMA. Sensitive continuum data reveal further fragmentation in five dusty cores at a resolution of several 10^3 AU. Spectral line emission from C18O, CH3OH, 13CS, H2CO and N2D+ are detected for the first time toward these dense cores. We found that three cores are chemically more evolved as compared with the other two; interestingly though, all of them are associated with collimated outflows as suggested by evidence from the CO, SiO, CH3OH, H2CO and SO emissions. The parsec-scale kinematics in NH3 exhibit velocity gradients along the filament, consistent with accretion flows toward the clumps and cores. The moderate luminosity and the chemical signatures indicate that the five cores harbor low- to intermediate-mass protostars that likely become massive ones at the end of the accretion. Despite the fact that the mass limit reached by the 1\sigma dust continuum sensitivity is 30 times lower than the thermal Jeans mass, there is a lack of a distributed low-mass protostellar population in the clump. Our observations indicate that in a protocluster, low-mass stars form at a later stage after the birth of more massive protostars.

Observational perspective of the youngest phases of intermediate-mass stars

Intermediate-mass protostars are the long-forgotten actors in the star-formation scenario. Historically, intermediate-mass stars have been associated to the low-mass regime and not too much attention has been drawn to them as a separate group. However, although their pre-main sequence evolution should be similar to that of low-mass protostars, intermediate-mass young stellar objects share many characteristics with their higher mass counterparts. This suggests that the star-formation mechanism could be similar for all luminosities.

Kinematics of powerful jets from intermediate-mass protostars in the Carina nebula

We present measurements of proper motions and radial velocities of four powerful Herbig-Haro (HH) jets in the Carina nebula: HH 666, HH 901, HH 902, and HH 1066. Two epochs of Hubble Space Telescope (HST) imaging separated by a time baseline of ~4.4 years provide proper motions that allow us to measure the transverse velocities of the jets, while ground-based spectra sample their Doppler velocities. Together these yield full three-dimensional space velocities. Aside from HH 666, their identification as outflows was previously inferred only from morphology in images. Proper motions now show decisively that these objects are indeed jets, and confirm that the intermediate-mass protostars identified as the candidate driving sources for HH 666 and HH 1066 are indeed the origin of these outflows. The appearance of two new knots in the HH 1066 jet suggest recent (~35 yr) changes in the accretion rate, underscoring the variable nature of accretion and outflow in the formation of intermediate-mass stars. In fact, kinematics and mass-ejection histories for all the jets suggest highly episodic mass loss, and point toward pronounced accretion fluctuations. Overall, we measure velocities similar to those found for low-mass protostars. However, the HH jets in Carina have higher densities and are more massive than their low-mass counterparts. Coarse estimates suggest that the heavy jets of intermediate-mass protostars can compete with or even exceed inject ~10 or more times the cumulative momentum injection of lower-mass protostars.

The hot core towards the intermediate-mass protostar NGC 7129 FIRS 2

NGC 7129 FIRS 2 (hereafter FIRS 2) is an intermediate-mass (2 to 8 Msun) protostar located at a distance of 1250 pc. High spatial resolution observations are required to resolve the hot core at its center. We present a molecular survey from 218200 MHz to 221800 MHz carried out with the IRAM Plateau de Bure Interferometer. These observations were complemented with a long integration single-dish spectrum taken with the IRAM 30m telescope. We used a Local Thermodynamic Equilibrium (LTE) single temperature code to model the whole dataset. The interferometric spectrum is crowded with a total of ~300 lines from which a few dozens remain unidentified yet. The spectrum has been modeled with a total of 20 species and their isomers, isotopologues and deuterated compounds. Complex molecules like methyl formate (CH3OCHO), ethanol (CH3CH2OH), glycolaldehyde (CH2OHCHO), acetone (CH3COCH3), dimethyl ether (CH3OCH3), ethyl cyanide (CH3CH2CN) and the aGg' conformer of ethylene glycol (aGg'-(CH2OH)_2) are among the detected species. The detection of vibrationally excited lines of CH3CN, CH3OCHO, CH3OH, OCS, HC3N and CH3CHO proves the existence of gas and dust at high temperatures. In fact, the gas kinetic temperature estimated from the vibrational lines of CH3CN, ~405 K, is similar to that measured in massive hot cores. Our data allow an extensive comparison of the chemistry in FIRS~2 and the Orion hot core. We find a quite similar chemistry in FIRS 2 and Orion. Most of the studied fractional molecular abundances agree within a factor of 5. Larger differences are only found for the deuterated compounds D2CO and CH2DOH and a few molecules (CH3CH2CN, SO2, HNCO and CH3CHO). Since the physical conditions are similar in both hot cores, only different initial conditions (warmer pre-collapse phase in the case of Orion) and/or different crossing time of the gas in the hot core can explain this behavior.

TheHerschel/HIFI spectral survey of OMC-2 FIR 4 (CHESS)

Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened up the terahertz regime to such surveys, giving access to the fundamental transitions of many hydrides and to the high-energy transitions of many other species. A comparative analysis of the chemical inventories and physical processes and properties of protostars of various masses and evolutionary states is the goal of the Herschel CHEmical Surveys of Star forming regions (CHESS) key program. This paper focusses on the intermediate-mass protostar, OMC-2 FIR 4. We obtained a spectrum of OMC-2 FIR 4 in the 480 to 1902 GHz range with the HIFI spectrometer onboard Herschel and carried out the reduction, line identification, and a broad analysis of the line profile components, excitation, and cooling. We detect 719 spectral lines from 40 species and isotopologs. The line flux is dominated by CO, H2O, and CH3OH. The line profiles are complex and vary with species and upper level energy, but clearly contain signatures from quiescent gas, a broad component likely due to an outflow, and a foreground cloud. We find abundant evidence for warm, dense gas, as well as for an outflow in the field of view. Line flux represents 2% of the 7 L_Sol luminosity detected with HIFI in the 480 to 1250 GHz range. Of the total line flux, 60% is from CO, 13% from H2O and 9% from CH3OH. A comparison with similar HIFI spectra of other sources is set to provide much new insight into star formation regions, a case in point being a difference of two orders of magnitude in the relative contribution of sulphur oxides to the line cooling of Orion KL and OMC-2 FIR 4.

HST/WFC3 imaging of protostellar jets in Carina: [Fe II] emission tracing massive jets from intermediate-mass protostars

We present narrowband WFC3-UVIS and -IR images of four externally irradiated protostellar jets in the Carina nebula: HH666, HH901, HH902, and HH1066. These massive jets are unusual because they are bathed in UV radiation from dozens of nearby O stars, but despite the strong incident ionizing radiation, portions of the jet remain neutral. Near-IR [Fe II] images reveal dense, neutral gas that was not seen in previous studies of H-alpha emission. We show that near-IR [Fe II] emitting gas must be self-shielded from Lyman continuum photons, regardless of its excitation mechanism (shocks, FUV radiation, or both). High densities are required for the survival of Fe+ amid the strong Lyman continuum from Tr14, raising estimates of the mass-loss rates by an order of magnitude. Higher jet mass-loss rates require higher accretion rates onto their driving protostars, implying that these jets are driven by intermediate-mass (IM; ~2-8 Msun) stars. Indeed, the IR driving sources of two of these outflows have luminosities that require IM protostars. All four of these HH jets are highly collimated, with opening angles of only a few degrees, similar to those observed in low-mass protostars. We propose that these jets reflect essentially the same outflow phenomenon seen in wide-angle molecular outflows associated with IM and high-mass protostars, but that the collimated atomic jet core is irradiated and rendered observable in Carina's harsh radiative environment. In more quiescent environments, this atomic core remains invisible, and outflows traced by shock-excited molecules in the outflow cavity give the impression that these outflows have a wider opening angle. Thus, the externally irradiated jets in Carina constitute a new view of collimated jets from IM protostars, and offer strong additional evidence that stars up to at least ~8 Msun form by the same accretion mechanisms as low-mass stars.

OH far-infrared emission from low- and intermediate-mass protostars surveyed withHerschel-PACS

OH is a key species in the water chemistry of star-forming regions, because its presence is tightly related to the formation and destruction of water. This paper presents OH observations from 23 low- and intermediate-mass young stellar objects obtained with the PACS integral field spectrometer on-board Herschel in the context of the Water In Star-forming Regions with Herschel (WISH) key program. Most low-mass sources have compact OH emission (< 5000 AU scale), whereas the OH lines in most intermediate-mass sources are extended over the whole PACS detector field-of-view (> 20000 AU). The strength of the OH emission is correlated with various source properties such as the bolometric luminosity and the envelope mass, but also with the OI and H2O emission. Rotational diagrams for sources with many OH lines show that the level populations of OH can be approximated by a Boltzmann distribution with an excitation temperature at around 70 K. Radiative transfer models of spherically symmetric envelopes cannot reproduce the OH emission fluxes nor their broad line widths, strongly suggesting an outflow origin. Slab excitation models indicate that the observed excitation temperature can either be reached if the OH molecules are exposed to a strong far-infrared continuum radiation field or if the gas temperature and density are sufficiently high. Using realistic source parameters and radiation fields, it is shown for the case of Ser SMM1 that radiative pumping plays an important role in transitions arising from upper level energies higher than 300 K. The compact emission in the low-mass sources and the required presence of a strong radiation field and/or a high density to excite the OH molecules points towards an origin in shocks in the inner envelope close to the protostar.