IRS5 Research Articles

FAST [Feii] Wind with a Wide Opening Angle from L1551 IRS 5

We present new velocity-resolved spectra of the [Fe II] λ1.644 μm line emission toward the L1551 IRS 5 outflow. The spectra were taken toward the bright [Fe II] knots PHK 1 and PHK 2 with the slit positions perpendicular to the northern jet. We have two major conclusions: (1) At PHK 1, located 12 away from the L1551 IRS 5 VLA sources, the spatial profile of the low radial velocity component at VLSR ~ -110 km s-1 shows two spatial subcomponents with their FWHMs of 083 and 284. The wide subcomponent has an wide opening angle of ~100°, which is consistent with the opening angle suggested by the broad velocity width of the narrow one. It favors the interpretation that both subcomponents of the low-velocity component arise from the same outflow. The gas corresponding to the wide subcomponent fills the space between the optical jet and the shell of the CO molecular outflow and may be sweeping up envelope material in the vicinity of the protostars. (2) At PHK 2, located ~42 away from the VLA sources, we confirmed that the northern jet has two radial velocity components: VLSR ~ -270 and -140 km s-1. The former velocity component is highly collimated because it has the same spatial width of 078 at both of the two [Fe II] knots.

Modelling the Dust Emission of the L1551 IRS5 Binary System

AbstractWe model the L1551 IRS5 source as a system containing two protostars, each surrounded by a circumstellar disk, both encircled by a circumbinary disk, and all disks surrounded by an extended infalling flattened envelope With this composite model, we can calculate self-consistently the spectral energy distribution of the source the shape of the ice and silicate features, and the spatial intensity distributions of the envelope and disks. We compare our model results with the observations, determining the physical parameters of the disks and the envelope. We find that flattened envelope collapse models are required in order to explain the observations and that the infall rate of the envelope is much larger than the accretion rates of the binary disks.

Radio Continuum Jets from Protostellar Objects

We have carried out a deep, 3.6 cm radio continuum survey of young outflow sources using the Very Large Array in its A configuration providing subarcsecond resolution. The eight regions observed are Haro 6-10 and L1527 IRS in Taurus, Haro 5a/6a in OMC 2/3, NGC 2023 MMS, NGC 2264 IRS1, HH 108 IRAS/MMS in Serpens, L1228, and L1251A. In combination with our similar and previously published maps of eight other star-forming regions, we find only one region with a single source, while the other 15 regions have on average 3.9 nearby sources. This supports the view that isolated star formation is rare. We have selected 21 objects, which are all young mostly Class I sources, and find a binary frequency of 33% in the separation range from 05 to 12''. This is within the uncertainties comparable to the observed binary frequency among T Tauri stars in a similar separation range. Seven of the 21 sources drive giant Herbig-Haro flows. Four of these seven are known to have companions (three are triple systems), corresponding to 57%. We discuss these results in relation to the hypothesis that giant Herbig-Haro flows are driven by disintegrating multiple systems.

Tracing the Shock Precursors in the L1448-mm/IRS 3 Outflows

We present the detection of the SiO ν = 0, J = 2-1, and J = 3-2 lines and of the HCO 101-000, J = 3/2-1/2, F = 2-1 line at ambient velocities toward the molecular outflows in L1448-mm and L1448 IRS 3. This is the first detection of HCO in a dark cloud. We have also measured lines of H13CO+, H13CN, HN13C, CH3OH, and N2H+. While the HCO and the SiO lines have the narrowest profiles with line widths of ~0.5 km s-1, the other lines have widths of ~1 km s-1. Toward L1448-mm, all lines except those of SiO and HCO show two distinct velocity components centered at 4.7 and 5.2 km s-1. HCO is only observed in the 4.7 km s-1 cloud and SiO in the 5.2 km s-1 component. The SiO abundance is ~10-11 in the 5.2 km s-1 clouds, 1 order of magnitude larger than in the 4.7 km s-1 component and in other dark clouds. The HCO abundance is ~10-11, similar to that predicted by the ion-molecule reactions models for the quiescent gas in dark clouds. The large change in the SiO/HCO abundance ratio (>150) from the 4.7 to the 5.2 km s-1 component, and the distribution and kinematics of the SiO emission toward L1448-mm suggest that the ambient SiO is associated with the molecular outflows. We propose that the narrow line widths and the abundances of SiO in the ambient gas are produced by the interaction of the magnetic and/or radiative precursors of the shocks with the clumpy preshocked ambient gas.

High Angular Resolution, Sensitive CSJ= 2–1 andJ= 3–2 Imaging of the Protostar L1551 NE: Evidence for Outflow‐triggered Star Formation?

High angular resolution and sensitive aperture synthesis observations of CS (J = 2-1) and CS (J = 3-2) emissions toward L1551 NE, the second brightest protostar in the Taurus molecular cloud, made with the Nobeyama Millimeter Array are presented. L1551 NE is categorized as a Class 0 object deeply embedded in the redshifted outflow lobe of L1551 IRS 5. Previous studies of the L1551 NE region in CS emission revealed the presence of shell-like components open toward L1551 IRS 5, which seem to trace low-velocity shocks in the swept-up shell driven by the outflow from L1551 IRS 5. In this study, significant CS emission around L1551 NE was detected at the eastern tip of the swept-up shell from Vlsr = 5.3 to 10.1 km s-1, and the total mass of the dense gas is estimated to be 0.14 ± 0.02 M☉. In addition, the following new structures were successfully revealed: a compact disklike component with a size of ≈1000 AU just at L1551 NE, an arc-shaped structure around L1551 NE, open toward L1551 NE, with a size of ~5000 AU, i.e., a bow shock, and a distinct velocity gradient of the dense gas, i.e., deceleration along the outflow axis of L1551 IRS 5. These features suggest that the CS emission traces the postshocked region where the dense gas associated with L1551 NE and the swept-up shell of the outflow from L1551 IRS 5 interact. Since the age of L1551 NE is comparable to the timescale of the interaction, it is plausible that the formation of L1551 NE was induced by the outflow impact. The compact structure of L1551 NE with a tiny envelope was also revealed, suggesting that the outer envelope of L1551 NE has been blown off by the outflow from L1551 IRS 5.

Adaptive Optics Spectroscopy of the [Feii] Outflow from DG Tauri

We present results of the velocity-resolved spectroscopy of the [Fe ii] � 1.644 lm emission line along the optical jet emanating from DG Tau. The slit spectrum, obtained with the Subaru Telescope adaptive optics system at an angular resolution of 0>16, shows strong, entirely blueshifted emission on the southwest side of the star. A faint, redshifted counterfeature was also detected on the northeast side with emission within 0>7 of the star being occulted by the circumstellar disk. The blueshifted emission has two distinct radial velocity components. The low-velocity component (LVC) has a peak radial velocity of �� 100 km s � 1 with a FWHM line width of � 100 km s � 1 , and it peaks at 0>2–0>5 from the star. The high-velocity component (HVC) peaks at 0>6–0>8 away from the star, showing a peak radial velocity of �� 220 km s � 1 with a line width of � 50 km s � 1 . These characteristics are remarkably similar to the [Fe ii] outflow from L1551 IRS 5, although the linear scales of the HVCs and LVCs are different for the two objects. We conclude, as an analogy to the case of L1551 IRS 5, that the HVC is a well-collimated jet launched from the region close to the star and that the LVC is a disk wind with a wide opening angle. Detailed comparison of emission parameters between the two sources, however, suggests that part of the LVC emission from DG Tau arises from the gas entrained and accelerated by the HVC, if we assume continuous steady state outflows. The presence of two distinct emission components clearly separated in space and velocity may favor theoretical models with two outflows: one is the LVC magnetohydrodynamically driven near the inner edge of an accretion disk, and the other is the HVC driven by the reconnection of dipolar stellar magnetic fields anchored to the disk. Subject headings: ISM: Herbig-Haro objects — ISM: individual (DG Tauri, HH 158) — ISM: jets and outflows — stars: formation — stars: pre–main-sequence — techniques: high angular resolution

The Binary Jet in L1551 IRS 5

We have used the Very Large Array with the Pie Town antenna of the Very Long Baseline Array to image for the first time L1551 IRS 5 at 3.5 cm with an angular resolution of ~01. These observations clearly reveal the presence of a binary jet, with each component approximately centered on the 7 mm compact protoplanetary disks previously reported. As anticipated from results at various wavelengths, L1551 IRS 5 is a binary system where each star is surrounded by a disk and drives a collimated jet. The curved morphology of the northern jet, showing reflection symmetry, is suggestive of that expected for a source in a binary orbit. A region of radio emission located between the two jets could be a zone of shock interaction between the two outflows.

Very Large Array Observations of Proper Motions in L1551 IRS 5

Using high angular resolution (~01) Very Large Array observations made at 2 cm during the period 1983 to 1998, we report the detection of proper motions in the components of the binary radio source in L1551 IRS 5. The absolute proper motions observed in these two protostars, of order 25 mas yr-1 or ~17 km s-1 at a distance of 140 pc, are very similar in magnitude and direction to those of T Tauri stars in the same region and are attributed to the large-scale motion of the parent molecular complex. The relative astrometry between the two components reveals orbital proper motions that suggest that the total mass and period of the binary system are ~1.2 M☉ and ~260 yr, respectively.

The Structure of Young Stellar Jets and Winds Revealed by High Resolution [Fe II] λ1.644μm Line Observations

We present high angular resolution spectra taken along the jets from L1551 IRS 5 and DG Tau obtained with the Subaru Telescope. The position-velocity diagrams of the [Fe II] λ 1.644 μm emission line revealed remarkably similar characteristics for the two sources, showing two distinct velocity components separated from each other in both velocity and space with the entire emission range blueshifted with respect to the stellar velocity. The high velocity component (HVC) has a velocity of -200 -300 km s-1 with a narrow line width, while the low velocity component (LVC) is around -100 km s-1 exhibitinig a broad line width. The HVC is located farther away from the origin and is more extended than the LVC. Our results suggest that the HVC is a well-collimated jet originating from the region close to the star, while the LVC is a widely-opened wind accelerated in the region near the inner edge of the accretion disk.

Orbital Proper Motions in the Protobinary System L1527/IRAS 04368+2557?

Using high angular resolution 7 mm Very Large Array (VLA) observations, we show that the low-mass protostellar object IRAS 043682557 in L1527 is composed of two radio sources separated in projection by 25 AU. One of the two components is extended in the direction perpendicular to the outflow powered by IRAS 043682557 and is, most likely, a compact accretion disk ( AU), similar to those found in L1551 R ∼ 20 IRS 5 by Rodro ´guez et al. As in L1551 IRS 5, the disk found here is small compared to those around T Tauri stars. Tidal interactions with nearby companions provide a natural way of truncating disks, and we argue that the two millimeter-wavelength sources in L1527 trace a compact binary system, where the disk surrounding one of the components has been truncated by the tidal influence of the other. A comparison between observations obtained in 1996 and 2002 reveals large proper motions, which can be only partly attributed to the overall largescale motion of the region in which IRAS 043682557 is located. The remaining “residual” proper motions might trace the orbital motion of the binary and would suggest a total mass for the system larger than 0.2 M, and likely of the order of 0.5–2 M,. This mass (only a small fraction of which is in the disk) is of the same order as that of the extended surrounding envelope of gas and dust traced by far-infrared and submillimeter observations ( M,), implying that the stars that will eventually form out of IRAS 043682557 would M ∼ 0.5 env have already acquired a significant fraction of their final masses. It is worth noting that multiepoch VLA studies of nearby protobinary systems similar to that presented here could provide direct mass estimates in most nearby star-forming sites. Combined with submillimeter observations of the surrounding envelopes, this would provide a more reliable measure of the evolutionary status of binary protostars. Subject headings: astrometry — binaries: general — ISM: jets and outflows — stars: formation

A Comprehensive Study of the L1551 IRS 5 Binary System

We model the Class I source L1551 IRS 5, adopting a flattened infalling envelope surrounding a binary disk system and a circumbinary disk. With our composite model, we calculate self-consistently the spectral energy distribution of each component of the L1551 IRS 5 system, using additional constraints from recent observations by ISO, the water ice feature from observations with SpeX, the SCUBA extended spatial brightness distribution at submillimeter wavelengths, and the VLA spatial intensity distributions at 7 mm of the binary disks. We analyze the sensitivity of our results to the various parameters involved. Our results show that a flattened-envelope collapse model is required to explain simultaneously the large-scale fluxes and the water ice and silicate features. On the other hand, we find that the circumstellar disks are optically thick in the millimeter range and are inclined so that their outer parts hide the emission along the line of sight from their inner parts. We also find that these disks have lower mass accretion rates than the infall rate of the envelope.

Velocity‐resolved [Feii] Line Spectroscopy of L1551 IRS 5: A Partially Ionized Wind under Collimation around an Ionized Fast Jet

We present a new [Fe II] λ1.644 μm emission-line image of the L1551 IRS 5 jets and a velocity-resolved spectrum obtained with the Subaru Telescope. In the [Fe II] line image showing two separate jetlike features, a strong and spatially wide [Fe II] emission feature was found elongated along the northern jet at its base. The echelle spectrum taken along this feature shows two prominent velocity components together with underlying pedestal and wing emissions. The entire emission range is blueshifted with respect to the systemic velocity, which shows that the emission originates from an outflow. The high-velocity component (HVC) at VLSR = -300 km s-1, which has a corresponding Hα emission, consistently shows a narrow line width of ~40 km s-1 (deconvolved FWHM value). The low-velocity component (LVC) at VLSR = -100 km s-1, on the other hand, has no corresponding Hα feature and is located spatially closer to the IRS 5 VLA sources (IRS 5 VLA) than is the HVC. The LVC shows broad line widths of 130-160 km s-1 (FWHM) near IRS 5 VLA, while its line width decreases with increasing distance from it. We interpret from these characteristics that the HVC is a spatially narrow, well-collimated ionized stellar jet and that the LVC is a widely opened, partially ionized disk wind that is being collimated as it travels away from the origin. The inclination-corrected velocity of the HVC, 440 km s-1, may imply that the accreting protostar driving the jet is rotating at almost its breakup speed. The LVC has an inclination-corrected wind velocity of ~200 km s-1, suggesting that it is accelerated at the region where the accretion disk is interacting with the stellar magnetic field. The pedestal and red wing features may represent the LVC gas entrained and accelerated by the HVC. It is possible that the two apparently parallel jetlike features seen more than 6'' away from the IRS 5 VLA sources are the brightened edges of the partially ionized wind.

Discovery of X-ray emission from the protostellar jet L1551 IRS5 (HH 154)

We have for the first time detected X-ray emission associated with a protostellar jet, on the jet emanating from the L1551 IRS5 protostar. The IRS5 protostar is hidden behind a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock “working surface”, i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is, at erg s-1, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with a temperature of 4 MK, higher than the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.

A multiwavelength study of the S106 region

The molecular cloud associated with Sharpless 106 has been studied in a variety of (sub)millimeter CO rotational lines on angular resolution scales from 11 00 to 80 00 . We used the KOSMA 3 m telescope to obtain an extended 12 CO J =3 !2 map, from which we calculate a total mass of 2000 M and an average density of 1:4 10 3 cm 3 for the molecular cloud. The peak intensity region around the massive young star S106 IR was observed in 13 CO J =6 ! 5a nd 3! 2w ith KOSMA and in isotopomeric low-J CO lines with the IRAM 30 m telescope. A clump decomposition made for several lines yields a common clump-mass spectral index of =1 :7, illustrating the self-similarity of the detected structures for length-scales from 0.06 to 0.9 parsec. All 12 CO and 13 CO line proles within approximately 2 0 around S106 IR show blue wing emission and less prominent red wing emission, partly aected by self-absorption in colder foreground material. We attribute this high-velocity emission to the ionized wind of S106 IR driving a shock into the inhomogeneous molecular cloud. We do not nd evidence for a smooth or fragmented disk around S106 IR and/or an expanding ring in the observed CO emission distribution. The excitation conditions along a cut through the molecular cloud/H II region are studied with an LTE analysis (and an Escape Probability model at the position of S106 IR), using the observed CO line intensities and ratios. The kinetic gas temperature is typically 40 K, the average density of the cloud in the core region is 9 10 3 cm 3 , and the local density within the clumps is 9 10 4 cm 3 .T he 13 CO/C 18 O line and column density ratios away from S106 IR reflect the natural isotopic abundance but towards the optical lobes and the cavity walls, we see enhanced 13 CO emission and abundance with respect to C 18 O. This shows that selective photo-dissociation is only important close to S106 IR and in a thin layer of the cavity walls. In combination with the results from the excitation analysis we conclude that the molecular line emission arises from two dierent gas phases: (i) rather homogeneous, low- to medium-density, spatially extended clumps and (ii) embedded, small (0.2 pc), high-density clumps with a low volume lling factor.

The molecular disk surrounding the protostellar binary L1551 IRS5

The inner three arcminutes surrounding the Class 0/1 binary protostar L1551 IRS5 have been observed using the J=1→0 transitions of the HCO+, H13CO+, 12CO and 13CO molecular species. Since the line core of HCO+ is self reversed over a substantial part of our map, observations of isotopomers such as H13CO+ are required in order to estimate the mass of the molecular gas in the immediate vicinity of IRS5. Our observations demonstrate the presence of a large ( ~ 7000 AU radius) dense, possibly rotating, molecular disk with a mass of a few M⊙ oriented perpendicular to the major axis of an extended molecular outflow. The disk is surrounded by an envelope with a radius of ~ 10 000 AU that contains two massive (each ~ 1 M⊙) clumps. One of these features appears to be kinematically disconnected from both the disk and the molecular outflow.

From Infall to Rotation around Young Stellar Objects: A Transitional Phase with a 2000 AU Radius Contracting Disk?

Evidence for a transitional stage in the formation of a low-mass star is reported, intermediate between the fully embedded and the T Tauri phases. Millimeter aperture synthesis observations in the HCO+ J = 1-0 and 3-2, HCN 1-0, 13CO 1-0, and C18O 1-0 transitions reveal distinctly different velocity fields around two embedded, low-mass young stellar objects. The 0.6 M☉ of material around TMC 1 (IRAS 04381+2517) closely follows inside-out collapse in the presence of a small amount of rotation (~3 km s-1 pc-1), while L1489 IRS (IRAS 04016+2610) is surrounded by a 2000 AU radius, flared disk containing 0.02 M☉. This disk shows Keplerian rotation around a ~0.65 M☉ star and infall at 1.3(r/100 AU)-0.5 km s-1, or, equivalently, sub-Keplerian motions around a central object between 0.65 and 1.4 M☉. Its density is characterized by a radial power law and an exponential vertical scale height. The different relative importance of infall and rotation around these two objects suggests that rotationally supported structures grow from collapsing envelopes over a few times 105 yr to sizes of a few thousand AU, and then decrease over a few times 104 yr to several hundred AU typical for T Tauri disks. In this scenario, L1489 IRS represents a transitional phase between embedded young stellar objects and T Tauri stars with disks. The expected duration of this phase of ~5% of the embedded stage is consistent with the current lack of other known objects like L1489 IRS. Alternative explanations cannot explain L1489 IRS's large disk, such as formation from a cloud core with an unusually large velocity gradient or a binary companion that prevents mass accretion onto small scales. It follows that the transfer and dissipation of angular momentum is key to understanding the formation of disks from infalling envelopes.

Twin, Aligned Protoplanetary Disks around the Components of the Young Binary System L1551 IRS5

VLA 7 mm continuum observations carried out by Rodríguez et al. (1998) with 0.06″ resolution toward L1551 IRS5 have shown two resolved dust sources. Both sources are elongated with almost the same orientation, similar flux densities, and similar angular sizes, being interpreted by these authors as a system of twin, aligned protoplanetary disks of 10 AU radius around the components of a young binary of 50 AU separation. These protoplanetary disks, the smaller ones ever imaged at mm wavelengths, are about a factor of ten smaller than disks around isolated stars, but have enough mass to form solar systems similar to our own. The fact that the disks are aligned with each other argues in favor for long-lived planetary systems in this binary. We all look forward to the future development of interferometers such as the EVLA, SMA, and ALMA in order to characterize the physical conditions of these associated protoplanetary disks by detecting spatially resolved molecular line emission. With these upcoming interferometers it will be possible to make great strides forward in our knowledge of the star formation processes.

Twisted Jets from L1551 IRS5

We have carried out high-resolution near-infrared imaging observations of a protostar L1551 IRS 5 with the Subaru Telescope. The jet structure of IRS5 is resolved into two independent jets for the first time from the ground. Successive near-infrared spectroscopy has revealed that the jet emission is dominated by [Fe II] lines in the J and H-bands. While the visual-extinction reaches more than 20 mag in the close vicinity of IRS 5, it decreases rapidly at ∼1″ from IRS 5 and remains constant around 7 mag at larger distances. The twisted structure and bright emission knots are intrinsic to the jets, not due to a spatial variation of the extinction.

A high-resolution radio survey of Class I protostars

We report the results of a survey of low mass Class I protostars in the cm continuum. In the initial survey, seven sources in the Taurus star formation were observed with the VLA at 0``.25 resolution. All seven sources drive CO outflows and display Herbig-Haro flows in the optical or near infrared wavebands. 4/7 sources were detected, two of which are new discoveries in systems of very low luminosity, one being the lowest luminosity system detected to date in the cm continuum. Notably, three sources were not detected to a 3-sigma limit of 0.10 mJy/beam, which indicates that significant cm continuum emission is not a universal feature of Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3-sigma limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the discriminating feature of the detected systems is a relatively high ionisation fraction in the stellar wind. Temporal variability of the outflow may also play a role. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is clearly resolved on a 0``.4 scale at 2 cm and 3.5 cm. Follow-up imaging with MERLIN did not detect this source with a 0``.04 beam, indicating that the radio emission is generated in a region with a radius of about 25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of near infrared data. Interpretation of this system is complicated by the existence of a quadrupolar outflow, which we originally detected through polarimetric imaging. We present a near infrared H2 image in which a bow shock in the secondary outflow is clearly seen. This complicated structure may have been caused by a gravitational interaction between two protostars.

A high-resolution radio survey of Class I protostars

We report the results of a survey of low-mass Class I protostars in the cm continuum. In the initial survey, seven sources in the Taurus star formation region were observed with the VLA at 0.25-arcsec resolution. All seven sources drive CO outflows and display Herbig-Haro flows in the optical or near-infrared (NIR) wavebands. Four out of seven sources were detected, two of which are new discoveries in systems of very low luminosity, one being the lowest luminosity system detected to date in the cm continuum. Notably, three sources were not detected to a 3σ limit of 0.10 mJy/beam, which indicates that significant cm continuum emission is not a universal feature of Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3σ limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the discriminating feature of the detected systems is a relatively high ionization fraction in the stellar wind. Temporal variability of the outflow may also play a role: only recently ejected knots may have sufficiently dense plasma to be optically thick to free-free emission, and hence produce detectable flux. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is clearly resolved on a 0.4-arcsec scale at 2 and 3.5 cm. Additional imaging with MERLIN did not detect this source with a 0.04-arcsec beam, indicating that the radio emission is generated in a region with a radius of ≈25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of NIR data. Interpretation of this system is complicated by the existence of a quadrupolar outflow, i.e. two bipolar outflows along roughly perpendicular axes, which we originally detected through polarimetric imaging. We present an NIR H2 image in which a bow shock in the secondary outflow is clearly seen. This complicated structure may have been caused by a gravitational interaction between two protostars.