OUTFLOWS AND YOUNG STARS IN ORION'S LARGE COMETARY CLOUDS L1622 AND L1634

John Bally; Josh Walawender; Bo Reipurth; S. Thomas Megeath

doi:10.1088/0004-6256/137/4/3843

1. INTRODUCTION

Collimated, bipolar outflows accompany the birth of young stars from the earliest stages of star formation to the end of their accretion phase (e.g., Reipurth & Bally 2001; Bally et al. 2006). Herbig-Haro (HH) objects, visual wavelength shocks powered by outflows from young stars, are a sensitive tracer of outflows that propagate in regions of low obscuration. HH objects trace shocks powered by ejecta from recent accretion events onto their source young stellar objects (YSOs). Thus, deep narrowband Hα and [S ii] imaging is a powerful tool for finding outflows and the young stars that power them.

The Orion superbubble contains dozens of cometary molecular clouds illuminated by OB stars in the Orion OB1 association (Alcala et al. 2008). Many exhibit ongoing star formation (Lee et al. 2005). Their investigation can provide important clues about a variety of processes such as the photoevaporation, acceleration, compression, and destruction of interstellar clouds located in close proximity to OB stars. Do radiation fields, winds, and supernova explosions trigger cloud formation or hinder it? Do these fields inhibit or promote star formation? In this paper, we present new evidence for recent star formation in two of the largest cometary clouds located in the interior of the Orion superbubble.

L1622 and L1634 are two parsec-scale cometary clouds in the interior of the Orion superbubble relatively close to Orion's OB association subgroups. They make excellent targets for the study of photoerosion, radiation-driven compression, and triggered star formation. In this paper, narrowband visual wavelength images, visual wavelength spectra, and broad-band Spitzer infrared images of these clouds are presented. These observations reveal new HH outflows and YSOs in these clouds.

L1622 is superimposed on the northern part of Barnard's Loop at the northeastern periphery of the Orion B molecular cloud about 40 pc in projection from the center of the OB1b subgroup of Ori OB1. It is the largest member of a family of cometary clouds located in the northeastern portion of Orion that includes the L1617 clouds that contain the famous HH 110 and 111 protostellar jets. The southwestern rim of the L1622 cloud appears to be irradiated by massive members of the 1b subgroup of the Orion OB1 association. Thus, the distance to this cloud has conventionally been assumed to be about 400 to 500 pc, the distance to the Orion molecular clouds and the OB association. However, analysis of the Tycho 2 photometry for Hipparcos stars toward L1622 (Knude et al. 2002) indicates the presence of a layer of dust at a distance of about 160 to 200 pc, well in front of the Orion OB1 association. Wilson et al. (2005) proposed a distance of 120 pc and used ¹²CO observations to estimate the mass of L1622 to be about 129 M_☉ for this distance. If L1622 were located nearly 200 pc in front of Orion's OB stars, the Hα emission at its southwestern rim would not have any good candidate illuminating sources. In this paper, it is argued that L1622 is located at approximately the same distance as Orion's OB stars. Kun et al. (2008) used stellar spectroscopy to arrive at the same conclusion and presented new CO data to estimate the cloud mass to be in the range 750 <M_☉< 1500 M_☉. Scaling the Wilson et al. (2005) mass estimate to 400 pc implies a mass of about 1.4 × 10³ M_☉. The ¹³CO survey of Bally et al. (1987) yields a mass of order 400 M_☉ for the more compact core region traced by visual wavelength extinction.

L1622 is actively forming young stars. LkHα 334 through 337 were first identified by Herbig & Kameswara Rao (1972). Subsequent studies revealed the Hα emission-line stars L1622-1 through L1622-16 (Ogura & Hasegawa 1983). For a recent review of the L1622 region, see Reipurth et al. (2008).

L1622 has been cited in the recent literature because it is one of the few, but growing number of, dark clouds that exhibits excess microwave emission from spinning dust grains (Casassus et al. 2006). This emission may be produced by very small grains, or electric dipole radiation from hydrogenated fullerenes (Iglesias-Groth 2006). The presence of such emission may be related to the irradiation of the dust in this cloud by a relatively strong UV radiation field that can charge grains by photoemission and spin them up by the formation and subsequent ejection of H₂ from the grain surface. These results provide further evidence that L1622 is embedded in the Orion superbubble and irradiated by the Orion OB association.

L1634 is a degree-scale cometary cloud located about 20 pc due west of the 1c and 1d sub-groups of Ori OB1. The mass of this bright-rimmed cometary cloud can be estimated from the CO maps of Wilson et al. (2005) and is found to be of order 200 M_☉ at an assumed distance of 400 pc. The HH 240/241 (Hodapp & Ladd 1995; Davis et al. 1997) outflow system contains a spectacular chain of bright near-infrared shocks emitting in H₂ and [Fe ii] and has been mapped in the J = 1–0 transition of CO with the BIMA interferometer (Lee et al. 2000). The powering source, IRAS 05173+0555, has a bolometric luminosity of 17 L_☉ (Reipurth et al. 1993) and may be a transition object between the Class 0 and Class I phases (Beltrán et al. 2002). A small cluster of four additional centimeter-wavelength radio sources mark the locations of other obscured YSOs (Beltrán et al. 2002). One of these, IRS7, drives a second HH flow that criss-crosses the eastern lobe of HH 240. This HH flow, discussed below, appears to be an extension of a putative H₂ jet emerging from IRS7.

Alcala et al. (2008) tabulate about a dozen T Tauri stars and candidate Class II YSOs in the immediate vicinity of L1634. Most are located east of the cometary molecular cloud and its Hα bright rim in a region devoid of molecular cloud material. Thus, star formation in the past occurred east of the current location of the cloud.

2. OBSERVATIONS

2.1. CCD Imaging

The first set of images was obtained on the nights of 2001 December 15, 17, and 19 with the NOAO Mosaic2 Camera at the f/3.1 prime focus of the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory. The Mosaic camera is 8192 × 8192 pixels (consisting of eight 2048 × 4096 pixel CCD chips) with a pixel scale of 0 farcs 26 pixel⁻¹ and a field of view 35 farcm 4 on a side. We used narrowband filters centered on 6569 Å and 6730 Å both with an FWHM of 80 Å for our Hα and [S ii] observations, respectively. For our continuum image we used the SDSS i' filter which is centered on 7732 Å with a FWHM of 1548 Å.

The observations presented here were part of a complete survey of the entire Orion A and B cloud complex. In order to increase our observing efficiency, we opted not to take a set of five images in the standard MOSDITHER pattern which is normally used to eliminate cosmic rays and the gaps between the individual chips in the Mosaic camera. Instead, we took single 600 s images through the Hα and [S ii] narrowband filters and a 180 s exposure in the SDSS i-band filter at each pointing. As a result of this observing strategy, most of our images contain chip gaps, resulting in a loss of approximately 3% of the area coverage at each pointing. However, for L1634, an extra image was obtained in each filter to fill in most of the chip gaps.

The L1622 cloud was re-observed with SuprimeCam at the prime-focus of the Subaru 8 m telescope on Mauna Kea on 2006 January 4. A series of six dithered 120 s exposures were obtained through a narrowband Hα filter to produce the image presented here.

Images were overscanned, trimmed, bias subtracted, and flat fielded (using dome flats) in the standard manner using the MSCRED package in IRAF. Cosmic rays were removed using CRNEBULA.⁴ Because we used single exposures in much of the survey, the images may contain a small number of cosmic rays which were not eliminated by the CRNEBULA task; however, we can safely distinguish the cosmic rays from objects by their morphology: cosmic rays have sharp edges and are often only one pixel in extent.

We used the procedures MSCCMATCH, MSCIMAGE, and MSCIMATCH to remove relative distortions, generate a single extension FITS image, and match the sky background between images. For the pointings for which we obtained multiple images in each filter in a dither pattern, we used the MSCSTACK procedure to combine all images in each filter into a single image which eliminates the gaps between CCD chips.

2.2. Visual Spectroscopy

Visual wavelength long-slit spectra of the shocks in L1622 and L1634 were obtained with the Double Imaging Spectrograph (DIS) at the f/10 Nasmyth focus of the 3.5 m reflector at the Apache Point Observatory on 2007 December 30. A 1 farcs 5 wide by 5' long slit was used with a grating providing a spectral resolution R = λ/Δλ≈ 5000. At each slit orientation three images with exposure times of 10 minutes were obtained.

2.3. Spitzer Space Telescope

Spitzer observations of L1622 using the IRAC and MIPS instruments were obtained on 2005 October 27 as part of Spitzer Space Telescope program, "An IRAC Survey of the L1630 and L1641 (Orion) Molecular Clouds," (PID 43, PI Giovanni Fazio), also see Megeath et al. (2009). The IRAC data were taken in the high dynamic range (HDR) mode with four pairs of 0.6 and 12 s exposures obtained at each map position. IRAC observations of L1634 were obtained on 2004 February 18 as part of Spitzer Space Telescope program, "Star Formation in Bright Rimmed Clouds," (PID 202, PI: Giovanni Fazio).

The data reduction and photometry extraction of the IRAC and MIPS Lynds 1622 data is described in detail in Megeath et al. (2009); here we provide only a brief overview. The standard BCD images were downloaded from the public archive at the Spitzer Science Center. The IDL based Clustergrinder package was used to mosaic images and to identify point sources in mosaics (Gutermuth et al. 2008). A custom photometry program written in IDL was used to measure the photometry of every stars in individual BCD images using an aperture of 2.5 pixels and a sky annulus spanning 2 farcs 5 to 7 farcs 2 data. The MIPS data were reduced with the Data Analysis Tool (Gordon et al. 2005). The Photvis program (Gutermuth et al. 2008) was used to find sources and the MIPS photometry was then extracted using the IDL implementation of DAOPHOT (Landsman 1993; Megeath et al. 2009). The L1634 images displayed in this paper are PBCD products downloaded from the public archive at the Spitzer Science Center.

3. RESULTS AND INTERPRETATION

3.1. The distances to the L1622 and L1634 clouds

As discussed in the introduction, the distance to L1622 has been the subject of debate. Knude et al. (2002) and Wilson et al. (2005) proposed a distance of about 100–200 pc, placing this cloud close to the near wall of the Orion–Eridanus bubble. If L1622 is located about 200 pc in front of the Orion OB association, it is hard to explain the surface brightness of the Hα rim surrounding this cloud. A constraint on the physical separation of L1622 from the OB stars responsible for the photoionization of the bright rim can be obtained from the emission measure derived from the Hα line brightness. We used the Southern H-Alpha Sky Survey Atlas (SHASSA) (Gaustad et al. 2001) to determine the Hα surface brightness and emission measure (EM). The SHASSA images show that the background-subtracted surface brightness of the L1622 bright rim is about 1.2 × 10⁴ Rayleighs (1 R = 10⁶/4π photons cm⁻² s⁻¹ sr⁻¹) which corresponds to an emission measure of EM = 5800 cm⁻⁶ pc.

In photoionization equilibrium, the Hα surface brightness of a D-type ionization front at a locally spherical surface depends only on the incident Lyman continuum flux and is independent of the local radius of curvature. The flux of Lyman continuum radiation (in units of ionizing photons s⁻¹ cm⁻²) from a star with Lyman continuum luminosity L(LyC) is F(LyC) = L(LyC)/4πD², where D is the distance from the star. A cloud having a sufficiently high density to trap a D-type ionization front will be surrounded by an expanding layer of photoablating plasma that completely absorbs this flux. Assuming that the ionization front has a radius r_I, and that freshly ionized hydrogen flows away from this front at a constant velocity c_II (approximately given by the sound-speed in the photoionized gas), the electron density decreases with distance r from the center of the cloud as n_e(r) = n_e(r_I)(r_I/r)², where n_e(r_I) is the density of electrons (and protons) at the ionization front. In photoionization equilibrium, the electron density n_e(r_I) and mass-flux at the base of the flow is regulated by the condition that the Lyman continuum flux is completely attenuated by recombined hydrogen between the ionization front and the ionizing star. The recombination rate per unit volume is given by n²_e(r)α_B where α_B ≈ 2.6 × 10⁻¹³ cm³ s⁻¹ is the case-B recombination coefficient for hydrogen at a temperature of 10⁴ K. Integrating the recombination rate per unit volume from r_I to infinity and setting this equal to the Lyman continuum flux results in F(LyC) = n²_e(r_I)α_Br_I/3 which gives n_e(r_I) as a function of D; thus, n²_e(r_I)r_I = 3L(LyC)/4πD²α_B. The ionization front will be trapped in the cloud if the neutral cloud density exceeds n_e(r_I) (as a D-type front); if the neutral density is lower, the ionization front will race through the cloud as an R-type front with a velocity faster than the sound speed. Several simplifying assumptions have been made in this derivation. First, the density and pressure gradient in the ablating plasma will tend to accelerate the photoablation flow, resulting in a slightly faster decline in electron density than the inverse-square dependence on the distance from the cloud center. This effect has been ignored. Second, it is assumed that the flow accelerates to a constant velocity instantaneously at the ionization front.

The emission measure EM at the projected edge of the spherical cloud (defined by the apparent location of the ionization front) can be obtained by integrating the quantity n²_e(r)dl along the line of sight where dl is an infinitesimal displacement along the line of sight. For an electron density profile which declines as the inverse square of the distance from the cloud center, this results in EM = ∫n²_e(r)dl = (π/2)bn²_e(b) where b is the minimum distance between the line of sight and the cloud center (the "impact parameter" of the line of sight). The peak flux occurs where b = r_I. Using the value of n²_e(r_I)r_I derived in the previous paragraph gives EM = (3/8)L(LyC)/α_BD² which is independent of the cloud radius.

The numerical coefficient in the formula above for EM is model dependent and can range from about 0.3 to 0.5. The coefficient 3/8 (= 0.375) takes limb-brightening into account under the assumption of a spherically divergent flow with a constant flow speed with no accommodation for the effects of finite resolution. The latter effect becomes important when the resolution associated with an observation is comparable to the ionization front radius; this is not a serious issue in the case of L1622.

Assuming that the interior of the Orion–Eridanus bubble is irradiated by about a dozen OB stars with a total Lyman continuum luminosity L(LyC) = 10⁵⁰ photons s⁻¹, the above estimate of the surface brightness of the L1622 rim implies a physical separation of the cloud from the illuminating source of about D = (3L(LyC)/8α_BEM)^1/2⩽ 30 pc. This is comparable to the projected separation of the L1622 cloud from the H ii regions associated with σ-Ori and NGC 2024 in the Orion B cloud, and the eastern end of Orion's Belt. If L1622 was closer than 200 pc from the Sun, its separation from Orion's massive stars would be much greater than 30 pc, and its Hα rim would be much dimmer than observed.

The surface brightness of the ionization front surrounding the eastern rim of the L1634 cloud is about 8 × 10³ Rayleigh, about 30% lower than the rim associated with L1622. The L1634 rim faces the Orion Nebula and the 1c subgroup of the Orion OB1c association that lies in the foreground. The projected separation between L1634 and Orion's Sword which contains the Orion Nebula and the NGC 1980 cluster about 30' south of the Nebula is about 4° or 29 pc. The projected distance between L1634 and the Sword and the estimated separation from the illuminating source based on the emission measure agree within a factor of 2 for L(LyC) = 10⁵⁰ photons s⁻¹. Note that this is about a factor of five greater than the estimated Lyman continuum luminosity of the Trapezium in the Orion Nebula, indicating that additional sources of UV luminosity are responsible for the Hα surface brightness observed in the interior of the Orion–Eridanus superbubble. This argument provides strong evidence that both L1622 and L1634 are located within the interior of the Orion–Eridanus bubble close to the Orion OB1 association.

Clouds such as IC 2118 near Rigel, thought to be located 150–200 pc in front of the Orion OB1 association at a distance of about 220 pc from the Sun are seen as much dimmer bright rims in the SHASSA images. The northeastern rim of IC 2118 has an Hα surface brightness of about 1 to 1.5 × 10³ Rayleigh, about an order of magnitude lower than the rims of L1622 and L1634, indicating that these clouds are at least a factor of three times farther from the source of UV illumination than L1622. Thus, the Hα surface brightness of the rim surrounding L1622 strongly argues for a distance of order 400 pc, the distance recently measured to the Orion nebula (Menten et al. 2007; Sandstrom et al. 2007; Hirota et al. 2007).

3.2. L1622

Figure 1 shows a color image of L1622 in which Hα is shown in blue, 4.5 μm emission is shown in green, and 8.0 μm emission is shown in red. Extended Hα emission mostly traces ionized hydrogen, excited either by UV radiation or by shocks; this band traces ionization fronts, irradiated outflows, and fast shocks. Extended 4.5 μm emission frequently traces shock waves propagating into molecular media. Thus, this band is an excellent tracer of highly embedded protostellar outflows. The extended 8 μm emission is thought to be produced by very small dust grains and polycyclic aromatic hydrocarbons (PAHs) stochastically heated by UV photons.

The locations of 28 Spitzer/IRAC detected YSO candidates from Reipurth et al. (2008) are marked; these were taken from Megeath et al. (2009). Sources with infrared excesses were identified using the methodology of Winston et al. (2007). In addition, protostars were also identified from their 4.5 and 24 μm photometry using the approach in T. Megeath et al. (2009, in preparation). The primary contamination is from background galaxies, particularly active galactic nuclei (AGNs). The contamination was minimized using the criteria described by Gutermuth et al. (2008). Asymptotic giant branch (AGB) stars with dusty envelopes are an additional source of contamination (E. Winston et al. 2009, in preparation); however, the level of contamination is expected to be small for L1622 which is at a Galactic latitude of −11°. Furthermore, AGB stars are characterized by weak 24 μm excess emission; such sources are not included in our YSO sample. We use several reference fields near the Orion clouds to assess the contamination of YSOs (Megeath et al. 2009). The estimated number of contaminating galaxies or AGB stars misidentified as YSOs in the L1622 field is less than 0.4.

Figure 1 also shows that the peak Hα emission from the photoionized rims of the cloud (blue) is systematically displaced toward the southwest, the direction of illumination, with respect to the 8 μm emission from PAHs. The most prominent 4.5 μm emission features produced by shocks are shown in green.

Figure 2 shows an Hα image of L1622 on which the 11 of 14 spectroscopically confirmed YSOs investigated by Kun et al. (2008) are superimposed (three of these YSOs lie outside the field of view shown). Ten of these spectroscopically confirmed YSOs coincide with Spitzer-identified YSOs. One of these sources, HBC 515, is a triple system, but is listed as a single YSO in the Spitzer list (#16). Thus, the number of YSOs in L1622 is at least 34. Figure 2 also shows the VLA source identified by Rodriguez & Reipurth (1994). Although this source lies in the northwest lobe of the HH 963 outflow discussed below, is it likely to be a background extragalactic source, possibly an AGN? A very faint point source appears in all the Spitzer images at the position of the VLA source. It is brightest at 3.6 μm but also detected at 24 μm; the colors and fluxes are similar to gas-rich galaxies. The detection of radio continuum suggests that it contains an active nucleus. The source IRAS 05522+0146 is probably identical to YSO candidate 5 in Reipurth et al. (2008). Comparison of Figures 1 and 2 shows that YSOs and YSO candidates are concentrated toward the southwest (bottom right) portion of the image near the apex of this cometary cloud. Several YSOs such as LkHα 334 and 337 lie beyond the edge of the cloud to the south and southwest.

The most conspicuous YSO in L1622, HBC 515, is located at the head of the cometary cloud facing the Orion OB1 association. A prominent reflection nebula opens toward the southwest. No obvious HH objects are found in or along the axis of the reflection nebula. However, just where the reflection nebula protrudes in front of the ionization front at the southwest head of L1622, there is a faint excess of [S ii] emission compared to Hα.

Figures 2 and 3 show Hα and [S ii] images of the L1622 cloud. The images reveal several new HH objects in the L1622 cloud in addition to the previously known HH 122 (Reipurth & Madsen 1989), the brightest visual wavelength shock in this cloud. HH 122 is remarkably different in Hα and [S ii] indicating large variations in the excitation conditions in this complex nebula.

**Figure 3.** Mosaic2 [S ii] image showing L1622. HH objects discussed in the text (circles) and the Hα emission lien stars (squares) studied by Kun et al. (2008) are marked. The white stripes trace the gaps between the individual CCD chips. Taken with the CTIO 4 m Blanco telescope.
Download figure:
Standard image High-resolution image

There are several faint HH objects north and west of HBC 515. HH 959 is a [S ii] dominated filament at P.A. ∼ 80° that is centered on a faint star located at α(2000) = 05:53:45.1, δ(2000) = 01:42:04. Faint [S ii] emission extends about 50'' south and 1 to 2' east of this feature. The brightest [S ii] knot at the east end of this complex is designated HH 960 in Table 1. The L1622 YSO #22, a young star with a disk in the tabulation of Reipurth et al. (2008), is located about 2 farcm 5 northeast–east of HH 960 and is associated with a small reflection nebula. An Hα knot is located about 4'' from the star on an axis aimed at HH 960. However, it is unclear whether this is a reflection nebula or a small shock. If it is a shock, it would make YSO #22 a likely driving source for HH 960.

Table 1. Catalog of HH Objects in L1622 and L1634

Object	J(2000)	Comments
122	05 54 38.5 +01 43 56	Bubble of H-alpha, complex structure in [S ii]
		HH 122 (Reipurth & Madsen 1989,
		ESO Messenger, 55, 32)
959	05 53 45.1 +01 42 04	[S ii] filament centered on star. Jet?
960	05 53 55.9 +01 41 54	NW of HBC 515
961	05 54 20.0 +01 48 42	Diffuse [S ii] blob
962	05 54 21.8 +01 44 01	[S ii] counterpart of bright IRAC band 2 shock
963 NW3	05 54 24.7 +01 47 34	[S ii] dominated bow on bipolar microjet axis
NW2	05 54 38.6 +01 46 11	Partial bow due N of HH 122; on axis of bipolar jet
NW1	05 54 49.2 +01 44 33	NW bow
star	05 54 56.9 +01 42 56	Source star: bipolar microjet
SE1	05 54 58.0 +01 42 44	Knot SE of microjet, on jet axis
SE2	05 55 03.5 +01 41 38	Jet segment
SE3	05 55 09.1 +01 40 28	SE bow 1
964	05 54 45.5 +01 53 13	NE facing Ha + [S ii] bow shock. Close to axis of HBC515
240 a1	05 19 46.2 −05 51 59	First [S ii] shock west of IRAS 01573-0555
240 a2	05 19 43.8 −05 51 49	Second shock complex, brightest knot
240 A	05 19 40.6 −05 51 42	Brightest known in HH 240
240 D	05 19 36.8 −05 51 16	Western end of HH 240 flow in Hα
IRAS	05 19 48.3, −05 52 07	IRAS 0157-0555, source of HH 240/241
241 a	05 19 56.2 −05 52 26	First visually bright knot in HH 241
241 e	05 20 04.5 −05 52 33	Eastern tip of Hα bow in HH 241
979 NW2	05 19 39.6 −05 48 39	Northwest end of IRS 7 flow
979 NW1	05 19 45.3 −05 50 21
IRS 7	05 19 51.6 −05 52 10	Source of HH 979
979 SE1	05 19 57.8 −05 53 21	Knot complex
979 SE2	05 19 59.5 −05 54 12	[S ii] filaments
979 SE3	05 20 08.8 −05 56 03	Faint, extended [S ii] knot
980 NW1	05 19 33.7 −05 44 03	NW end of HH 980
Source	05 19 45.6 −05 46 38	Reflection nebula containing YSO
980 SE1	05 19 47.8 −05 47 25	Bright part of [S ii] filament
980 SE2	05 19 51.3 −05 47 51	Center of [S ii] filament
980 SE3	05 19 56.8 −05 48 14	East end of [S ii] filament
980 SE4	05 19 56.8 −05 48 14	SE tip of Hα bow shock

Download table as: ASCII Typeset image

HH 961 is a compact blob of pure [S ii] emission. Although it is located close to the axis of the giant HH 963 flow (see below), it has a very different degree of excitation. While all of the HH 963 components are Hα dominated, HH 961 has no associated Hα emission. The nearest YSO is #7 in the tabulation of Reipurth et al. (2008). This object is extremely red, being invisible in the i band (Figure 4) and below the limiting magnitude of the Two Micron All Sky Survey (2MASS). It is visible in the IRAC bands and brightest at 24 μm.

**Figure 4.** Mosaic2 SDSS i-band image showing L1622. Reflection nebulae discussed in the text are marked with ovals whose major axes indicate their orientation. Squares mark the locations of *Spitzer*-detected candidate YSOs. Taken with the CTIO 4 m Blanco telescope.
Download figure:
Standard image High-resolution image

HH 962 is a very faint [S ii] structure associated with a bright Spitzer band 2 (4.6 μm) shock complex that has a striking east-to-west oriented S-shaped symmetry centered on a bright knot (Figure 5). This feature is located 6 farcm 5 in projection from HBC 515 at P.A. ≈ 52° in the direction opposite to the reflection nebula illuminated by HBC 515. The Spitzer 24 μm images show a point source (#10) at α(2000) = 05:54:24.3, δ(2000) = 01:44:19 which looks like a compact bipolar nebula in the IRAC bands that is embedded within the band 2 nebular complex. It is likely that this YSO is the driver of this highly embedded and collimated outflow.

**Figure 5.** *Spitzer* 4.5 μm image showing HH 122 and 962 on a logarithmic intensity scale. HH objects discussed in the text are marked with circles. Squares show the *Spitzer*-selected candidate YSOs. Extended 4.5 μm emission is a good tracer of shock waves powered by protostellar outflows.
Download figure:
Standard image High-resolution image

HH 964 is a compact [S ii] dominated feature located about 16 farcm 6 northeast from HBC 515. In the Subaru Hα image, faint tendrils of emission extend for several arcminutes toward the southwest, giving the impression that the brightest portion of HH 964 marks the tip of a bow shock. The Spitzer IRAC band 2 images show a compact but faint emission feature. Additional, very faint band 2 emission features at the limit of sensitivity are located at α(2000) = 05:54:37.3, δ(2000) = 01:52:42 and α(2000) = 05:54:30.0, δ(2000) = 01:52:18. If these are real, then together with HH 964, they point back toward the binary YSO consisting of #27 and #28.

HH 963 is a highly collimated HH flow from a naked star located at α(2000) = 05:54:56.9, δ(2000) = 01:42:56 several arc minutes south of the southern rim of the L1622 cloud and about 13 farcm 5 east and 2 farcm 5 north of HBC 515 (Figures 6 and 7). This star is designated source #4 in Figure 1. The star is one of the reddest sources in the Spitzer IRAC images in the entire L1622 field and is associated with a 10'' long microjet extending toward the northwest that is dominated by [S ii] emission. That the driving star is clearly seen on the visual wavelength images indicates that it is not embedded in a resolved, opaque cloud core. The star may be surrounded by a small disk or the image trace an unresolved reflection nebula in a compact core.

**Figure 6.** Subaru Hα CCD image showing the various components of the HH 963 jet along with HH 122. The suspected source star is marked.
Download figure:
Standard image High-resolution image

**Figure 7.** Mosaic2 [S ii] CCD image taken with the CTIO 4 m Blanco telescope showing the various components of the HH 963 jet along with HH 122. The suspected source star is marked. Gaps between the 8 CCDs in Mosaic are responsible for the blank stripes.
Download figure:
Standard image High-resolution image

The blueshifted side of the HH 963 jet is oriented toward the northwest at P.A. ∼ 308°. Three faint bow shocks, NW1, 2, and 3 (see Table 1), face away from the star and are located at projected distances of 2 farcm 4, 5 farcm 6, and 9 farcm 3. At least three features are seen toward the southeast. HH 963 SE1 is a faint, unresolved Hα and [S ii] knot about 20'' southeast of the source. HH 963 SE2, located about 2 farcm 6 southeast of the source is a prominent Hα bright, arcminute long jet segment that is completely detached from the source. About 3 farcm 9 southeast from the star and downstream from the jet segment, there is a relatively bright Hα dominated bow, SE3 in Table 1. This outflow has a total projected length of about 13 farcm 2 or about 1.5 pc at an assumed distance of 400 pc. The northern lobe as traced by the three bow shocks NW1 through NW3 indicate a mild deflection of the flow toward the south. Thus, the HH 963 jet and chain of associated shocks join the growing list of parsec-scale outflows. This system is unusual in that it is powered by an unobscured star.

The moderate resolution DIS spectra show that the northwestern lobe is blueshifted with a radial velocity of about − 35 to −50 km s⁻¹ compared to the background Hα and [S ii] emission produced by the diffuse emission associated with the Orion superbubble. The detached jet segment on the southeast side of the source star has no discernible Doppler shift while the bow at the end of the southeast lobe is redshifted by about 25–40 km s⁻¹.

The source star exhibits strong forbidden [O i], [N ii], [S ii], and Hα emission. The stellar Hα profile extends over a velocity range of about ±325 km s⁻¹, and has an equivalent width in excess of 10 Å, indicating that it is a classical T Tauri star.

IRAS 05522+146 is located about 5' north of the driving source of HH 963 and is associated with YSO candidate #5. Although the IRAS source coordinates are about 10'' west of the YSO, this is within the errors of the IRAS coordinates for faint sources in the presence of strong diffuse emission.

Several other YSOs in L1622 produce faint reflection nebulae (see Table 2 and Figure 4) in addition to HBC 515 (which is designated YSO #16 in Figure 1). A compact group of three stars consisting of YSOs #19, 20, and 21 illuminate a complex reflection nebula; the northwesternmost star in the triple appears to be associated with a cavity that opens toward the northwest. However, most of the scattered light is located north and northeast of this star system. A secondary cavity appears within this component about 20'' to the north. YSO #22 illuminates a small reflection nebula that opens toward the southwest. YSOs #27 and 28 illuminate a faint i-band nebula that opens toward the southwest at P.A. ∼ 240°. Finally, YSO #9, the northernmost YSO in L1622, opens toward P.A. ∼ 320° to the northwest. There is a faint hint of nebulosity in the opposite direction indicating the possible presence of a disk shadow.

Table 2. Reflection Nebulae in L1622 and L1634

Object	J(2000)	Comments
L1622
HBC 515	05 54 03.2 +01 40 26	P.A. ∼ 235
YSO #22	05 54 04.5 +01 42 58	P.A. ∼ 235
YSO #19, 20, 21	05 54 19.9 +01 42 57	P.A. ∼ 310
YSO #27, 28	05 54 26.6 +01 52 15	P.A. ∼ 330
YSO #9	05 54 36.3 +01:53:55	P.A. ∼ 320
L1634
IRAS 05173 − 0555	05 19 48.3, −05 52 07	P.A. ∼ 75/255 (Spitzer only)
IRS 7	05 19 51.6 −05 52 10	P.A. ∼ 135
L1634 N	05 19 45.6 −05 46 38	P.A. ∼ 100 (i band)
L1634 N		P.A. ∼ 290 (4.6 μm)

Note. Object designations such as YSO #22 etc are taken from Reipurth et al. (2008).

Download table as: ASCII Typeset image

4. L1634

The L1634 cloud, located four degrees west of the Orion Nebula, contains a spectacular bipolar outflow emerging from IRAS 05173 − 0555, a 17 L_☉ Class I protostar with a sub-millimeter luminosity, L_sub-mm = 0.13 to 0.68 L_☉. De Vries et al. (2002) present single dish maps of the L1634 cloud, the dense core containing IRAS 05173 − 0555, and the outflow emerging from it. They measured a core mass of about 12 M_☉ from N₂H⁺ and 28 M_☉ from HCO⁺. Lee et al. (2000) present interferometric maps of CO emission from the outflow with arcsecond resolution, and showed that the western lobe associated with HH 240 is blueshifted while the eastern lobe associated with HH 241 is redshifted. The total outflow mass inferred from the CO data is about 0.7 M_☉. Beltrán et al. (2002) measured a core mass of 3 to 10 M_☉ and a mass infall rate of $\dot{M}_i \approx 2.6$ to 8 × 10⁻⁵M_☉ yr⁻¹. If such a high accretion rate were to fall directly onto the star, the accretion luminosity would be much higher than the observed value. Thus, Beltrán et al. (2002) conclude that this accretion must be onto the circumstellar disk. Seale & Looney (2008) present a Spitzer image of the immediate vicinity of IRAS 05173 − 0555 that shows a reflection nebula with a narrow opening angle. Interestingly, the orientation of the reflection nebula differs from that of the outflow by about 15°.

A second embedded YSO, IRS 7, located about 1' east of IRAS 05173 − 0555 has L_sub-mm ≈ 0.03 L_☉. It too drives a highly collimated outflow, first detected in the 2.12 μm line of H₂, at about a 30° angle with respect to HH 240/241 (Davis et al. 1997) which corresponds to P.A. ≈ 130°/310°. Beltrán et al. (2008) present high angular resolution VLA NH₃ maps of the cores containing IRAS 05173 − 0555 and IRS 7.

The western lobe of the IRAS 05173 − 0555 outflow contains the shocks collectively known as HH 240A through D while the eastern lobe contains HH 241A through D, with the shocks closest to the IRAS source designated A (Cohen 1980; Bohigas et al. 1993). Davis et al. (1997) show that in the 2.12 μm line of H₂, HH 240 consists of a set of quasi-periodically spaced bow-shocks. Lee et al. (2000) presented interferometric maps of J = 2-1 CO emission from the HH 240/241 outflow, finding that it has a mass of about 0.2 M_☉ of accelerated gas. These authors also show a close correspondence between high-velocity CO features and shocks seen in visual and near-IR images.

The Mosaic images show the HH 240/241 flow in detail (Figures 8 and 9). The eastern end of the IRAS 05173 − 0555 outflow, where it breaks out of the L1634 cloud about 1' east of H₂ knot HH 241D, consists of a spectacular fragmented bow shock in both Hα and [S ii]. The tips of at least three bows are cleanly traced by Hα; [S ii] is predominantly seen in the bow shock wings where the excitation is presumably lower while Hα traces the bow tips and wings. H₂ knots HH 241C and A are both associated with bright Hα emission. The brightest visual wavelength HH object in the eastern lobe of the IRAS 05173 − 0555 flow is located about 15'' west of H₂ knot HH 241A; this is the first shock that is visible in Hα or [S ii] east of the source.

**Figure 8.** Mosaic2 Hα image showing L1634. HH objects discussed in the text are marked. Arrows indicate the locations and orientations of the three major outflows. The thick diagonal streak is a satellite trail.
Download figure:
Standard image High-resolution image

**Figure 9.** Mosaic2 [S ii] image showing L1634. HH objects discussed in the text are marked. Arrows indicate the locations and orientations of the three major outflows.
Download figure:
Standard image High-resolution image

The western lobe of this outflow is even brighter at visual wavelengths. A collection of faint [S ii] knots first appears about 40'' west of the IRAS source. The brightest feature in this outflow is the strong emission associated with H₂ knot HH 240A where the flow appears to be deflected by about 10° toward the north. This outflow is at least 7' long (0.8 pc long at an assumed distance of 400 pc).

Two other large but fainter HH flows are detected in the L1634 cloud. The infrared source IRS 7 drives a flow that crosses the HH 240/241 system at an angle of about 30°–35°; it has a position angle P.A. ≈ 130°/310°. A faint chain of reflection nebulae visible in both the narrow- and i-band images indicates the flow orientation. This flow was discovered by Hodapp & Ladd (1995) who found faint H₂ emission (their knots 4 and 9) along the axis of the reflection nebula. Two arcminute-scale [S ii] dominated filaments trace the northwestern lobe of this flow, HH 979 NW1 and HH 979 NW2. A compact cluster of [S ii] knots appears in the southeastern lobe about 30'' beyond where the IRS 7 flow crosses HH 241. This feature is designated HH 979 SE1. A faint group of [S ii] filaments is located about 0.5 to 1' farther southeast, HH 979 SE2. Finally, an isolated knot of [S ii] emission marks what appears to be the last shock in the IRS 7 flow about 1' beyond the Hα rim that marks the eastern edge of the L1634 cloud and is designated HH 979 SE3. The IRS 7 flow may be 11' (1.6 pc at D = 400 pc) long if this southeastern feature is confirmed to be an HH object.

A compact reflection nebula located near the northern rim of L1634 appears to mark the location of the driving source of the third major outflow in this cloud. The reflection nebula, located at α(2000) = 05:19:45.4, δ(2000) = −05:52:10.3, opens toward position angle P.A. = 110° (Figure 10). Although there is no IRAS source at this location, there is a red 2MASS star with magnitudes m_J = 16, m_H = 15, and m_K = 14 at this location. The source star is here designated L1634N. The CO and HCO⁺ maps presented by De Vries et al. (2002) show a compact core at the location of the reflection nebula, but no analysis of its properties is given.

**Figure 10.** Mosaic2 SDSS i-band image showing L1634. HH objects discussed in the text are marked. The ellipses mark the locations and approximate orientations of reflection nebulae in the *Spitzer* images.
Download figure:
Standard image High-resolution image

The outflow from L1634N is indicated by a series of [S ii] dominated filaments that skirt the northeastern edge of the L1641 cloud as indicated by Hα emission and i-band structure. The flow emerges at P.A. ≈ 120°. The centers of these filaments are labeled SE1 and SE2 in Figures 8 and 9. These features appear to trace the southern rims of a pair of giant but faint bow shocks located southeast of the source whose tips are designated HH 980 SE1 and HH 980 SE2. The southern rim of these shocks is seen clearly in the [S ii] image. A [S ii] dominated knot is located on the opposite side of the bow shocks along the expected counterflow axis and is designated HH 980 NW1. The L1634N flow is at least 9' (1.3 pc at D = 400 pc) long.

The Spitzer Space Telescope observed the L1634 region using IRAC (Program ID 202; PI: Giovanni Fazio). The core containing IRAS 05173 − 0555 and IRS 7 were observed in all 4 IRAC bands; the core containing L1634N was observed only in bands 2 and 4. The IRAC images reveal prominent infrared reflection nebulae associated with all three YSOs. While both the IRS 7 and L1641N reflection nebulae are visible at visual wavelengths, the nebula associated with IRAS 05173 − 0555 only becomes visible at infrared wavelengths. Figure 11 shows an IRAC channel 2 (4.5 μm) image of the main YSOs in L1634. Figure 12 shows a color composite of the 3.6, 4.5, and 8.0 μm images.

**Figure 11.** *Spitzer* 4.5 μm image showing the L1634 cloud cores containing IRAS 05173 − 0555, IRS 7, and L1634N. The HH 240/241 outflow from IRAS 05173 − 0555 exhibits strong emission while several shocks associated with the IRS 7 flow are also visible. The extended 4.5 μm emission traces shocks powered by embedded outflows.
Download figure:
Standard image High-resolution image

**Figure 12.** *Spitzer* IRAC image showing 3.6 μm (blue), 4.5 μm (green), and 8.0 μm (red) images. Known YSOs are marked; "7" is IRS 7, "IRAS" is IRAS 05173 − 0555, and "N" is the new YSO L1634N. The various VLA sources are marked. VLA1 is a background galaxy. The extended green emission tracer shocks powered by outflows. The extended red emission tracers PAH emission associated with photon-dominated regions at the irradiated molecular cloud surface.
Download figure:
Standard image High-resolution image

As discussed by Seale & Looney (2008), the IRAS 05173 − 0555 IR reflection nebula is misaligned from the HH 240/241 outflow axis by about 20°–25°. Inspection of the Spitzer/IRAC images indicates that the outflow cavity may be illuminated asymmetrically; the south rim of the west-facing outflow cavity associated with the blueshifted flow, and the north rim of the east-facing cavity associated with the redshifted flow are nearly an order of magnitude brighter than the opposite walls of the cavity. If the orientations of the outflow axis and IR reflection nebula define the opening half-angle of the cavity, then it has an opening angle of about 50°. The cause of the brightness asymmetry is unclear. The NH₃ maps of Beltrán et al. (2008) show hints of a wide angle cavity, especially toward the east and a ridge of dense gas adjacent to the western side of the shocks associated with the outflow from IRS 7.

IRS 7 is associated with a prominent bipolar reflection nebula having an orientation similar to that of the HH 979 outflow. There are hints of an X-shaped geometry suggestive of a limb-brightened biconical cavity with an opening angle of about 40°. IRAC band 2 (4.6 μm) emission is associated with both HH 979 NW1 and SE1. However, in addition to the visual wavelength emission, the IRAC images show several knots of emission two-thirds of the way from IRS 7 to NW1. The morphology suggests that this IR emission is tracing shocks located in the wings of a bow shock whose high-excitation tip coincides with NW1.

The IR reflection nebula associated with L1634N extends predominantly toward the northwest of the illuminating YSO. In contrast, in the [S ii], Hα, and i-band images, the reflection nebula extends east of the YSO. Taken together, the visual and near-IR reflection nebulae appear to delineate the oppositely directed walls of an outflow cavity whose axis roughly coincides with that of the outflow traced by HH 980.

5. DISCUSSION AND CONCLUSIONS

Large format focal plane arrays on 4 m class telescopes have led to the discovery of ever fainter and larger outflows by means of the shock radiation that they produce. Prior to this study, both L1622 and L1634 were known to contain a few outflows and young stars; L1622 was known to contain HH 122, the reflection nebula associated with HBC 515, and several LkHα emission line stars. L1634 was known to contain the HH 240/241 outflow which has spectacular shock-excited H₂ emission, several embedded infrared and radio continuum sources, and Hα emission line stars east of the illuminated cloud edge. The Spitzer Space Telescope has increased the number of candidate YSOs in L1622 to at least 34. These stars tend to be concentrated near the high-extinction cores in the cloud. At least a dozen shock complexes associated with about a half dozen distinct outflows have been detected thus far. In most cases, candidate sources are apparent in the Spitzer data.

HH 963 is a parsec-scale flow consisting of a bipolar quasiperiodic chain of bow shocks that originates from a red but unobscured star south of the main body of L1622. The driving star exhibits many of the spectral characteristics of a T-Tauri star such as strong Hα emission and infrared excess. The star exhibits a bipolar high velocity microjet. This flow is highly unusual in several respects. First, the star is "naked" and not embedded in a resolved cloud core; it must be surrounded by a compact disk or envelope. Second, the southeastern lobe of the outflow contains an isolated jet segment. This segment and the Hα dominated bow shocks may be rendered visible by the external UV radiation field of the Orion OB association that permeates the region. Thus, HH 963 joins the growing list of irradiated HH outflows.

Although HH 962 is only faintly detected in [S ii], it is the most spectacular outflow in L1622 in the 4.5 μm Spitzer/IRAC band 2 image which is a good tracer of shocks propagating in molecular media. This flow appears to be powered by a highly embedded IR source about which it exhibits S-shaped symmetry.

The visually spectacular HH 122 is associated with faint knots of Spitzer/IRAC band 2 emission. Several faint [S ii] dominated filaments and knots, HH 959 through 961, are located along the southwest rim of the L1622 cloud.

IRAC detected 28 candidate YSOs, and Kun et al. (2008) confirmed the YSO nature of 10 of these objects as well as an additional four stars in the vicinity of L1622 (LkHα334, 337, L1622-3, and L1622-15), bringing the total number of young stars in L1622 to at least 34 when the triple nature of HBC 515 is included. Some of these stars may be multiple, potentially increasing this number. Additionally, there may be some additional young stars that do not exhibit infrared excess emission detectable by Spitzer. Assuming that there are 34 YSOs in L1622, that the median masses of these stars is 0.6 M_☉, and cloud mass of 400–1500 M_☉, the star formation efficiency of order 1–5%. Thus, this cloud has been very inefficient in forming stars. Most YSOs are located near the southwestern head of this cometary cloud. A few (LkHα 334, 337, and #4—the source of the HH 963 outflow) are located beyond the present cloud edge toward the south and west. It is impossible to determine whether photoerosion caused the cloud edge to retreat toward the northeast, or because the stars moved from their birthplaces toward the southwest.

Kun et al. (2008) estimate that the median ages of the YSOs in this cloud are about 1 Myr. Older stars formed from this cloud may have lost the obvious signatures of youth and have therefore missed being counted. If such older YSOs exist, then the above estimate of the star formation efficiency is a lower bound.

The L1634 cloud west of the Orion Nebula is shown to contain at least three HH outflows. New visual wavelength features of the well known HH 240/241 outflow from IRAS 05173 − 0555 include a fragmenting bow shock located at the eastern terminus of this outflow where its redshifted lobe is punching through the ionization front which wraps around L1634, and faint shocks in the blueshifted western lobe located between the IRAS source and the bright shocks in HH 240A. The HH 979 outflow from IRS 7 which was previously only detected in near-IR H₂ emission is shown to contain several faint HH objects. The HH 979 outflow crosses over and is at least 1.5 times longer than the HH 240/241 flow. The innermost HH objects, NW1 and SE1 are shown to be associated with faint IRAC band2 emission. NE1 in particular appears to be a molecular hydrogen dominated bow shock that has associated visual wavelength emission only at its northwestern tip. Finally, a previously unknown YSO embedded in a condensation in the northern part of L1634 is associated with a reflection nebula and a large but very faint outflow, HH 980, whose lobes emerge into the photo-ionized environment of L1634. The southern rims of several large bow shocks provide the highest surface brightness features in this outflow. The flow from L1634N joins the growing list of protostellar outflows propagating in ionized media.

There are at least five radio continuum sources in or near the southern core in L1634 (Beltrán et al. 2002). The data presented here demonstrate that VLA1 is associated with a background galaxy seen through translucent portions of L1634. Of the remaining VLA sources VLA4 and 5 lie beyond the edge of the cloud. No IRAC source is detected at the position of VLA5 and no i-band sources are seen at these locations. Thus, we conclude that these objects are not young stars. VLA2 is located in an obscured portion of the cloud with no IRAC counterpart while VLA3 coincides with IRAS 05173 − 0555. Thus, the main portion of L1634 contains at least three YSOs: IRAS 05173 − 0555, IRS 7, and L1634N. Three additional Hα emission line stars are located west of the ionization front (Alcala et al. 2008). There are four stars with visual magnitudes between 8 and 9 located west of the ionization front that wraps around the main core. However, at least three of these are foreground objects based on their parallax, spectral types, and magnitudes. HD 34835, a B8 star a few arcminutes east of the main L1634 ionization front, may be close to the distance of L1634. The brightest portion of the L1634 ionization front, the cloud edges as imaged in the i-band, and 8 μm emission detected by Spitzer are located near this star. Thus, there are at least six YSOs in or near L1634. Although HD 34835 does not exhibit any signs of youth, this roughly 3 M_☉ star might have been formed in L1634. Assuming a mass of 200 M_☉ for L1634, and that there are six or seven YSOs with a median mass of 0.6 M_☉ implies a current star formation efficiency of only about 2%, indicating very inefficient star formation.

To be classified as a YSO, stars either have to be embedded, drive outflows, be classified as Hα emitters, or exhibit infrared excess having the colors of a circumstellar disk. Stars exhibit these properties for about 1 to 2 Myr. If L1622 and L1634 last longer than this timescale, then our estimates of the star formation efficiency are lower bounds. More detailed searches for YSOs may reveal additional young stars which formed from these clouds, potentially increasing the estimated star formation efficiency.

This paper made use of data from the Southern H-Alpha Sky Survey Atlas (SHASSA), which is supported by the National Science Foundation (Gaustad et al. 2001). We thank Maria Kun for stimulating the discussion of the distance to L1622 at the ESO workshop "Star Formation Across the Milky Way Galaxy" held in Santiago, Chile in March 2008. J. Bally acknowledges support by NASA grant NNA04CC11A to the CU Center for Astrobiology and NSF grant AST0407356. JW and BR were supported by the NSF through grants AST-0507784 and AST-0407005. B. Reipurth was partly supported by the NASA Astrobiology Institute under Cooperative Agreement No. NNA04CC08A issued through the Office of Space Science. This work is based on observations made with the Spitzer Space Telescope (PID 43), which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for S. T. Megeath was provided by NASA through contracts 1256790, 1281300, and 1289605 issued by JPL/Caltech. We thank the referee, Dr. William Henney, for valuable comments that improved the manuscript.

OUTFLOWS AND YOUNG STARS IN ORION'S LARGE COMETARY CLOUDS L1622 AND L1634

Article metrics

Permissions

Author e-mails

Author affiliations

Dates

ABSTRACT

1. INTRODUCTION