EDP Sciences
Free Access
Issue
A&A
Volume 546, October 2012
Article Number A74
Number of page(s) 41
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201219131
Published online 10 October 2012

Online material

Appendix A: The bright-rim cloud BRC13 and vicinity

BRC13 (Sugitani et al. 1991) is the textbook example of a bright-rimmed cloud: it resembles a finger pointing to HD 18326, the exciting star of W5-E. An IR source IRAS 02570+6028 lies at the tip of the finger, inside the rim; its luminosity is 1440 L according to Sugitani et al. (1991; from IRAS fluxes) or 490 L according to Morgan et al. (2008; from SCUBA fluxes); both estimates are for a distance of 2 kpc.

Figure A.1 shows the morphology of BRC13. The head of BRC13 is symmetric (over a length of 1.7 pc) with respect to the direction of HD 18326, the exciting star of W5-E. This is strong evidence that the ionizing radiation of HD 18326 has shaped this structure. Figure A.1 shows that a small IR cluster lies inside BRC13, at its tip. This cluster is likely the counterpart of IRAS 02570+6028. According to KOE08 it contains several Class II and Class I YSOs. Sources #1 and #2 in Table A.1 have been identified by KOE08 as Class I YSOs. YSO #2 has

thumbnail Fig. A.1

Bright-rimmed cloud BRC13. Top: composite colour image: red is the column density map from Herschel data, green is the Herschel-PACS image at 100 μm, and blue is the DSS2-red image. The peak column density is 1.7 × 1022 cm-2. The contours correspond to column densities of 1/2, 1/4, and 1/8 of the peak value. The yellow line shows the direction of the exciting star HD 18326. The field size is 14.7′  ×  7.5′. North is up and east is left. Bottom right: the cluster inside BRC13: red is the Spitzer emission at 4.5 μm, green is the 2MASS K stellar emission, and blue is the Hα emission from the ionized boundary layer (image obtained at the 1.2-m telescope of the Observatoire de Haute-Provence). Bottom left: identification of the YSOs discussed in the text and present in the KOE08 catalogue. The grey-scale underlying image is the Spitzer 4.5 μm data. The red and green circles are respectively the proposed Class I and Class II YSOs. The blue arrows point to the Hα emission stars (classical T Tauri-like stars) identified by Ogura et al. (2002).

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Table A.1

Photometry of point sources: the bright-rim cloud BRC13 and vicinity.

no detectable counterpart at Herschel wavelengths. Source #3 (identified in Fig. A.1) is not present in the catalogue of KOE08; however it is probably also a Class I YSO: it strongly resembles YSO #1 as both have 24 μm and 100 μm counterparts. The brightest source at 24 μm is a Class II source, BRC13-a, located at the center of a region of extended emission visible in Hα and at all Spitzer wavelengths. It is this YSO and the extended emission region (plus probably the nearby Class I YSOs #1 and #3) that are seen in the PACS and SPIRE images. At these wavelengths we are unable to separate the central object from its associated extended emission zone. We have measured the fluxes of this extended source using apertures of diameter 30″, 45″, and 60″ (respectively one, two, and three asterisks in Table A.1); the first aperture is that used by Morgan et al. (2008) to measure the SCUBA 450 μm and 850 μm fluxes, but only the last aperture covers the entire extended emission zone. We have used the same apertures to measure the fluxes in the Spitzer bands (asterisks). We list these measurements in Table A.1.

Figure A.1 shows the dense molecular material present at the head of BRC13, around BRC13-a. The diameter at half intensity of the molecular condensation is 0.31 pc. Its peak column density is N(H2) = 1.7 × 1022 cm-2, thus a density n(H2) ~ 1.7 × 104 cm-3. Its temperature, estimated from the Herschel fluxes, is in the range 24.4 K to 27.8 K (the smallest aperture giving the hotter temperature; Table 5). This differs slightly from the temperature of 23 K obtained by Morgan et al. (2008). The mass of this condensation is estimated to be  ~6.1 M in the 30″ diameter aperture and increases to 21.8 M in the 60″ diameter aperture (considering only the Herschel data; Table 5). (from SCUBA data Morgan et al. estimate a mass of 35 M in the 30″ central aperture, corrected for a distance of 2 kpc and a dust opacity at 850 μm of 1.25 cm2 g-1). We suggest that this condensation is heated from the inside by the nearby Class II object BRC13-a; this YSO is possibly partly embedded and appears to have carved a cavity open in our direction. This has created a path of low extinction allowing us to see BRC13-a and the light reflected by the borders of the cavity, even at optical wavelengths.

Another fainter condensation (called b) is also present along the axis of BRC13, at the back of the brightest condensation, and thus farther away from the exciting star (Fig. A.1). We measured its Herschel fluxes using an aperture of diameter 90″ (measurements in Table A.1). The mass of this condensation is estimated to be 55 M for a temperature of 19.5 K found in the temperature map (Fig. 3). The column density decreases along the axis when going away from the tip of the rim. Dense material is also present on the south border of BRC13 (and not at its north border, a deviation from axial symmetry). The observed column density is not very high, of the order of 3 × 1021 cm-2. This material is probably made of material collected during the expansion of W5-E. Alternatively it may be part of a east-west pre-existing filament as shown by Fig. 4. The column density is not high enough for star formation to proceed via the collect and collapse process, and indeed no YSOs are detected there.

At Hα wavelength BRC13 appears as an extinction region surrounded by a bright rim. Therefore the molecular condensation inside the rim, responsible for the extinction, is located slightly in front of the ionized region. The NVSS image shown by Morgan et al. (2004) shows faint radio-continuum emission from the very head of BRC13 (the ionized boundary layer seen also in Hα); they estimated the electron density there to be  ~250 cm-3. Morgan et al. (2004) have shown that the pressure in the ionized boundary layer is much higher than the pressure in the enclosed molecular material. (This is confirmed by the Herschel observations.) Thus a D type ionization front progresses inside the BRC, compressing the molecular material and potentially triggering star formation. The observed IR cluster possibly results from this triggering.

Another signature of triggered star formation in BRC13 is the small-scale sequential star formation observed in this object. Numerous Hα emission line stars have been detected in the vicinity of BRC13 by Ogura et al. (2002). These stars are YSOs, pre-main sequence stars in the T Tauri/Herbig AeBe phase. They are identified on Fig. A.1. They are located near the tip of the bright rim, at the head of the rim, or just outside the rim. Most of these emission line stars are also Class II YSOs in KOE08. The youngest Class I YSOs are located inside the bright rim. This suggests small-scale sequential star formation, with star formation proceeding from the side of the exciting star outward away from the central H ii region. Chauhan et al. (2011a) used deep V,I photometry to estimate the age of the Hα emission stars; they obtained different ages, 2.44 ± 1.37 Myr and 1.61 ± 1.41 Myr respectively for 24 stars outside the rim and 10 stars inside the rim. This confirms the small-scale sequential star formation (however the large scatter in the stellar ages is worrisome).

Appendix B: The bright-rim cloud BRC14 and vicinity

BRC14 is another bright-rimmed cloud catalogued by Sugitani et al. (1991). It harbours the bright IR source IRAS 02575+6017 (also AFGL 4029, Price & Walker 1976). The luminosity of this IRAS source is  ~1.9 × 104 L (Snell et al. 1988; corrected for a distance of 2 kpc). Beichman (1979) has shown that AFGL 4029 is composed of two mid-IR sources, IRS1 and IRS2, separated by 22″.

IRS1 and IRS2 are radio sources, respectively G138.295+1.555 and G138.300+1.558 (Kurtz et al. 1994, Zapata et al. 2001). Both are also small Hα emission regions (see Fig. 1 in Deharveng et al. 1997). G138.300+1.558 is a thermal H ii region harbouring a small cluster dominated by a B1V star (Deharveng et al. 1997; their star #26, affected by a visual extinction  ~8 mag). IRS1 is associated with a YSO and its reflection nebula (Deharveng et al.; their star #25, affected by a visual extinction  ~30 mag). A high velocity optical jet originates from IRS1 (Ray et al. 1990). The Spitzer images confirm the presence of two bright IR components, a cluster around a bright star in the direction of IRS2, and a bright YSO associated with a reflection nebulosity in the direction of IRS1. IRS1 has been observed at high resolution in the 8 μm−13 μm range by Zavagno et al. (1999) and at 24.5 μm by de Wit et al. (2009). De Wit et al. estimate a flux density of 680 Jy for IRS1 from the Spitzer-MIPS observations at 24 μm.

thumbnail Fig. B.1

Identification of the YSOs in the BRC14 field. The grey-scale image shows the Herschel-PACS 100 μm emission. The three red boxes show the same zone, displayed with different intensity cuts to allow the identification of all the central sources.

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The bright-rimmed cloud BRC14 borders a massive molecular cloud. This cloud has been mapped in 12CO(1−0) by Loren and Wootten (1978), Snell et al. (1988), Carpenter et al. (1990; 2000), Karr & Martin (2003). High resolution maps have been obtained by Niwa et al. (2009; HPBW = 15.6″) in 13CO(1−0) and C18O(1−0). The condensation enclosed by BRC14 is their clump 7, the brightest one, with a peak column density in the range 5.8−9.4 × 1022 cm-2 (depending on the transition), and a mass of 740 M.

A CO outflow was detected by Snell et al. (1988) in the direction of AFGL 4029 (size  ~0.55 pc; age  ~1.7 × 105 yr). Their resolution (HPBW ~  45″) is not sufficient to associate it definitively with IRS1 or IRS2 (see Fig. 2 in Deharveng et al. 1997). An H2O maser has been detected in the direction of IRAS 02575+6017 by Churchwell et al. (1990); its velocity, VLSR = −33.3 km s-1, indicates that it is associated with the region. Once again the resolution of the observations do not allow a direct association with IRS1. Methanol maser emission at 6.7 GHz has been searched for but not detected in the direction of the IRAS source (Slysh et al. 1999).

Small-scale sequential star formation is also observed in the direction of BRC14 (Matsuyanagi et al. 2006; Chauhan et al. 2011a, and references therein). Using deep photometry Chauhan et al. (2011a) estimated a mean age of 1.01 ± 0.73 Myr for 18 YSOs on or inside the rim, and an age of 2.32 ± 1.22 Myr for 58 sources outside the rim.

B.1. YSOs in the field of BRC14

The Herschel observations show a bright massive condensation enclosed by BRC14, and numerous point-like sources. These objects are identified in Fig. B.1, listed in Table B.1, and commented on below. We show a few SEDs in Fig. B.2. The SEDs have been fitted allowing the interstellar (external) extinction to range between 1.96 mag and 8 mag.

  • BRC14-a: this source is observed in the direction of three nearbystellar sources (separated by some 6″), all classified as Class I by KOE08. At least one of them (a3) is a 24 μm source. BRC14-a is likely associated with it. If true, only one model satisfies χ2 − χ2(best) per data point  ≤3. This model corresponds to an intermediate mass Stage I YSO. But the SED is not well constrained, perhaps because of the multiplicity of the central object.

  • BRC14-b: a bright 24 μm source lies in this direction. It belongs to a small group of stars, observed at Spitzer-IRAC wavelengths; an object is clearly dominant at 8.0 μm and 24 μm. The SED is that of a Stage I YSO of intermediate mass. The parameters of the disk are not well constrained.

  • BRC14-c: this source is faint and observed in the direction of a rather faint 24 μm source. This source was identified by KOE08 as a “deeply embedded protostar”. We confirm here that its SED is that of a Stage I YSO of intermediate mass.

  • BRC14-d and -e: these sources, separated by some 5″, lie near the bright central IRS sources. They cannot be separated at SPIRE wavelengths. Only one 24 μm source is observed in the direction of BRC14-e; BRC14-d does not have any 24 μm counterpart and thus is a candidate Class 0 object. For BRC14-e, six models satisfy χ2 − χ2(best) per data point  ≤3. They point to a Stage I YSO of intermediate mass; but here again the parameters of the disk are not well constrained.

  • BRC14-f: this source is observed in the direction of a Class II YSO according to KOE08. Because it lies near IRS1, its fluxes are difficult to measure. Its 24 μm counterpart has not been measured by KOE08, and our measurement at 100 μm is very uncertain. At Spitzer-IRAC wavelengths this YSO is the brightest source of a small group of stars. The SED is not well constrained; the Herschel source is possibly not linked to the central YSO (but adjacent to it).

  • BRC14-g, -h, -i: these sources lie near each other on the sky. BRC14-g and -h are possibly slightly extended. This region is complicated, as a small cluster is present in its direction. One IRAC Class II source is observed in the direction of BRC14-h (called h1 in Fig. B.3), which has no 24 μm counterpart. Two Class II sources are observed in the direction of BRC14-g (called g1 and g2 in Fig. B.3), and only g1 has a 24 μm counterpart. Another 24 μm source lies between -g and -h, that is not in KOE08 catalogue. It is difficult to associate -g and -h with any of these sources. For example, Fig. B.2 shows that if we associate BRC14-h to the Class II YSO h1 the SED is not well fitted at Herschel wavelengths. The situation is the same for BRC14-g. This and the fact that the Herschel sources are slightly extended suggest either that the cluster with its three Class II YSOs is embedded in (or lie adjacent to) the extended g and h condensations, or that it contains several Class 0 sources. BRC14-i has no IRAC counterpart, and has possibly a very faint 24 μm extended counterpart; it is a candidate Class 0 source.

  • BRC14-j: this 100 μm source has no IRAC or 24 μm counterpart. It is a candidate Class 0 source.

Several other sources are Class I YSOs according to KOE08. YSOs #1 and #2 have very faint 100 μm counterparts, at the limit of detection (Fig. B.1). Their SEDs are not at all well constrained, as shown by Fig. B.2. However, the first 20 best fits for YSO #2 have all an envelope accretion rate equal to zero. Thus, YSO #2 is probably a Class II YSO, of intermediate mass; the parameters of its disk are not constrained. The SED of YSO #1 resembles that of YSO #2; it is also probably a Class II YSO (in contrast to the finding of KOE08). The four other Class I YSOs (according to KOE08; #17122 close to BRC14-i, #17029, #17037, and #17076, close to the central IR sources) have no detectable or measurable 100 μm counterpart. We cannot estimate their evolutionary stage.

Table B.1

Photometry of point sources: the bright-rim cloud BRC14 and vicinity.

Table B.2

Characteristics of the YSOs: the bright-rim cloud BRC14 and vicinity.

thumbnail Fig. B.2

Spectral energy distributions of a few point sources in the BRC14 field. We used the SED fitting tool of ROB07.

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thumbnail Fig. B.3

The BRC14-g, -h, -i, and -j 100 μm point sources and their associated mid-IR cluster. The black crosses show the position of the three Class II YSOs identified by KOE08.

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The cloud enclosed by BRC14 contains in its center two very bright Herschel sources, c1 and c2, observed in the direction of IRS1 and IRS2. They are discussed in the following section.

B.2. The dense condensation enclosed by BRC14

thumbnail Fig. B.4

Bright-rimmed cloud BRC14. The underlying greyscale image is the Spitzer 3.6 μm emission showing the cluster AFGL4029 (IRS2) and the YSOs BRC14-c1 (IRS1; a Class I source according to KOE08)). The red contours correspond to column densities of 0.5, 1, 2.5, and 5 × 1022 cm-2. The peak column density is  ~1.0 × 1023 cm-2. The arrow points to the exciting star HD 18326. The blue line shows the position of the IF. North is up and east is left.

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The molecular condensation enclosed by BRC14 is conspicuous in the Herschel images. It is the brightest and most massive condensation of the observed field (Fig. B.4). It belongs to the east-west filament extending toward Sh 201. It is symmetric with respect to the direction of HD 18326. It contains the two bright IR sources IRS1 and IRS2.

The morphology of this condensation varies with the wavelength. At 100 μm it contains two slightly extended sources, observed in the direction of IRS1 and IRS2 (Fig. B.1); we call them c1 and c2 in Table B.1. The western one (IRS1) is the brightest (there is a factor of 3.5 in intensity between their peak emission values). At 160 μm, IRS2 becomes fainter; the ratio of the peak intensities is 2.85. At SPIRE wavelengths IRS2 is barely detected, IRS1 is dominant, but the peak emission is shifted toward the north-east; the column density peaks some 8″ (0.08 pc) north east of c1. The IR sources c1 and c2 are very bright 24 μm sources. They have been classified as Class I YSOs by KOE08, but no 24 μm measurements are given in the catalogue; c2 is the brightest at 24 μm, the contrary of what is observed at 100 μm and at longer wavelengths. This indicates that IRS2 is the hottest of the two sources. This is expected as IRS2 is not a single source, but corresponds to dust heated by the cluster exciting a compact H ii region (Deharveng et al. 1997).

This condensation has a peak column density N(H2) = 1.0 × 1023 cm-2, and a half-intensity diameter of 0.27 pc; this indicates a density n(H2) ~ 105 cm-3. We have measured the flux of this condensation through circular apertures of radius 40″, 50″, and 60″, centered on the column density peak at α(2000) = 03h01m31s79, δ(2000) =  + 60°29′21″. We give these measurements in Table B.1, under the labels c*, c**, and c***, respectively for the three apertures. The temperatures we derive from the Herschel fluxes are in the range 25.0 K to 26.4 K (the smallest aperture is the hottest), and the masses are in the range 170−250 M respectively for the smallest to the largest aperture (see Table 5). This can be compared to the results obtained by Morgan et al. (2008) from SCUBA data; a temperature of 27 K, a diameter of 0.4 pc, and a mass of 94 M for integration in a 30″ aperture (if corrected for a distance of 2 kpc and an opacity at 850 μm of 1.25 cm2 g-1).

Appendix C: The Sh 201 bipolar H II region and vicinity

C.1. Presentation of the region

Sh 201 is a small H ii region located to the east of W5-E, about 7.6 pc away from the IF bordering BRC14. It is coincident with the IRAS source IRAS 02593+6016, which has a luminosity of  ~1.1 × 104 L. It probably belongs to the same complex as W5-E since its velocity (V(RRLs) = −34.6 km s-1, Lockman 1989; V(Hα) = −35.5 km s-1, Fich 1990) is very similar to that of W5-E (V(H158α) = −37.0 km s-1, Dieter 1967; V(Hα) = −39.0 km s-1; V(Hα) = −34.3 km s-1, Mampaso et al. 1987).

Sh 201 is a bipolar optical H ii region of low excitation and low density, except for a bright Hα knot on the east side (Mampaso et al. 1987 and Fig. C.1). These authors suggest that this bright optical structure results from the interaction of the ionized gas with an adjacent dense molecular cloud. Sh 201 is a thermal radio source with an integrated flux density of 1.2 ± 0.2 Jy at 6-cm (Felli et al. 1987, and references therein). At a distance of 2 kpc this flux density indicates an ionizing photons flux of 4.25 × 1047 s-1 (using Eq. (1) in Simpson & Rubin 1990). This ionized flux points to an ionizing star of spectral type O9−O9.5V, if single, according to the calibration of Martins et al. (2005). Felli et al. (1987), Fich (1993), and Ojha et al. (2004) all show high-resolution radio continuum maps of Sh 201. These maps, obtained at different frequencies, are very similar to one another; this confirms the thermal nature of the radio-continuum emission. The radio source has a bright arc shaped edge on the east side, and a smoothly decreasing surface brightness on the other side (Fig. C.1). All aforementioned authors interpret this arc-shaped structure as a result of the interaction between the H ii region and an adjacent molecular cloud (on the east side). Felli et al. (1987) have shown that an O9 ZAMS star (the exciting star of the H ii region) placed at a distance of 0.26 pc (for d = 2 kpc) of the border of the cloud is consistent with the available data. This model does not attempt to explain the bipolar nature of Sh 201. Figure C.1 summarises the situation for the ionized gas: 1) the regions of radio and Hα emission have a similar extent, except for the central absorption zone; 2) the arc-shaped bright radio feature corresponds to the bright Hα knot; they are bordering a region of high extinction to the east of Sh 201; 3) a faint secondary radio component is present about 1 pc west of the brightest radio component (on the opposite side at the waist of the bipolar nebula).

thumbnail Fig. C.1

Emission of the ionized gas in Sh 201. The contours of the radio-continuum emission, as given by Ojha et al. (2004; their Fig. 6), are superimposed on a grey image of the Hα emission (DSS-2 red image). The brightest emission comes from the dense ionized layer bordering a molecular condensation. North is up and east is left.

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Ojha et al. (2004) discuss the stellar content of Sh 201, based on near-IR observations (see also Carpenter et al. 1993). A cluster, containing more than a hundred stars, is present in the direction of IRAS 02593+6016; it contains YSOs identified by their near-IR excess. The brightest and most reddened source is, according to these authors, an O6−O8 type star. Two massive stars with spectral type probably earlier than B2 are also present, which display a near-IR excess. These stars are identified on Fig. C.2; they are respectively #2 (the exciting star), #1 and #3.

thumbnail Fig. C.2

Young stellar objects and stars located at the waist of Sh 201. They are identified on the Spitzer-IRAC 4.5 μm image. Star #2 is the exciting star.

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thumbnail Fig. C.3

Bipolar nature of Sh 201. Red is for the column density map; the red contours are of column density; the peak column density is 6.2 × 1022 cm-2 (east condensation) and the contours levels are 0.1, 0.25, 0.5, 1, 2.5, and 5 × 1022 cm-2. Turquoise is for the Spitzer 8.0 μm image showing the PAH emission from the vicinity of the ionization front. The coordinates are J2000 in degrees.

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thumbnail Fig. C.4

Vicinity of Sh 201 and BRC14. Composite colour image with the Herschel-SPIRE 500 μm image in red, the Herschel-PACS 100 μm image in green and the Spitzer-MIPS 24 μm image in blue (all in logarithmic units). Red is the emission of the cold dust; it shows the dense material in this region. Green traces the emission from warmer dust in the PDRs of the H ii regions. Blue traces the emission from even hotter dust, located in the PDRs but also inside the ionized regions. YSOs appear as point sources at 24 μm and 100 μm. The field is 32.5′ × 21.8′. North is up and east is left.

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The molecular content of the region has been the subject of various studies. Martin & Barrett (1978) have detected and mapped the 13CO emission of a flat molecular cloud, located at the waist of the bipolar nebula, that is probably responsible for the optical absorption observed there (see also Fig. 3 in Felli et al. 1987). This cloud extends east-west, with a size of more than 18 pc  ×  6 pc, and has a velocity similar to that of the H ii region (V(CO) = −39.1 km s-1, Blair et al. 1975; V(CO) = −40 km s-1, Blitz & Fich 1982; V(CO) = −38 km s-1, Wooterloot & Habing 1985). Large-scale CO maps show the extent of this filament which goes from BRC14, through Sh 201, to eastern regions far away from Sh 201; its total east-west extent is  ~20 pc (Carpenter et al. 2000; Karr & Martin 2003; Niwa et al. 2009, their cloud 7). This filament has two condensations on each side of Sh 201 (clumps 8 and 9 in Niwa et al. 2009). The CS map of Carpenter et al. (1993) shows that the east clump is by far the brightest. It is also the densest of all the CO clumps located in the periphery of W5, with a density of 2.6 × 104 cm-3 (Niwa et al. 2009). It is the dense ionized layer bordering this clump that appears as the bright arc-shaped structure in the radio-continuum map and in Hα.

The Sh 201 region has been observed by Spitzer (KOE08). The 8.0 μm IRAC image, dominated by the PAH emission bands from the PDR, gives the best illustration of the bipolar nature of Sh 201. Figure C.3 shows the column density contour levels (obtained by Herschel) superimposed on the 8.0 μm image. It confirms: 1) the presence of a dense neutral filament extending east-west, with two bright condensations at the waist of the bipolar nebula and a hole of emission in the central region (occupied by ionized material); 2) the bipolar nature of Sh 201: two lobes extend perpendicular to the filament, up to 3.5 pc from the central exciting cluster.

Figure C.4 displays the distribution and temperature of the dust observed by Herschel. Red is 500 μm band that traces the emission of cold dust. As such, this channel shows the numerous cold filaments containing most of the mass, especially the east-west filament mentioned earlier that extends from BRC14 to Sh 201. Green is the 100 μm band, which traces the emission of warmer dust along the PDRs of the ionized regions. We see very clearly the two lobes perpendicular to the east-west filament, and a brighter emission at the waist of the bipolar nebula. Blue is the Spitzer 24 μm band, which traces even hotter dust. Its emission is located predominantly either in the PDRs or in the ionized region; we find 24 μm extended emission in the direction of the “interior” of W5-E and in the central region of Sh 201, close to the exciting cluster. Several cold dust condensations (red colour) are found along the east-west filament, one enclosed by BRC14, two on each sides of the waist of Sh 201 (the eastern one the brightest), and a small one at the extreme east,  ~5.5 pc from Sh 201 (called filament #1 in Sect. 6.6 and Fig. 15). The eastern condensation at the waist of Sh 201 has a peak column density of 6.2 × 1022 cm-2, corresponding to a visual extinction of 66 mag. The mass of this condensation, integrated over an area limited by the contour of 0.6 × 1022 cm-2 (10% of the peak intensity) is 235 M. This indicates a mean density of 3.5 × 104 H2 cm-3, in rather good agreement with the results of Niwa et al. (2009); they find from C18O observations of their clump 9 (corresponding to our eastern condensation) a column density of 4.3  ×  1022 cm-2, and a density of 2.6 × 104 cm-3. If we consider the contour at half-intensity as the integration limit, we measure a mass of 70 M for a size (diameter) of 0.25 pc. This indicates a core density of the order of 9 × 104 H2 cm-3, in good agreement with the results of Carpenter et al. (1993) who showed with their CS (2−1) observations that this condensation has a dense core.

thumbnail Fig. C.5

Young stellar objects detected at 100 μm in the vicinity of Sh 201. They are identified on the Herschel-PACS 100 μm image. (The contrast has been adjusted to better identify the sources.) The sources s1 and s2 are identified on Fig. C.7. The coordinates are J2000.

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The western condensation at the waist of the bipolar nebula has a smaller column density of 1.1 × 1022 cm-2 at the emission peak. It is slightly elongated north-south along the ionization front, with an equivalent diameter of 0.36 pc at half intensity; thus a smaller density of the order of 104 cm-3. Note that this condensation is not clump 8 in Niwa et al. (2009).

C.2. Young stellar objects in the vicinity of Sh 201

Table C.1

Photometry of point sources in the vicinity of Sh 201. The asterisks indicate our 24 μm measurements.

Table C.2

Characteristics of the YSOs in the vicinity of Sh 201.

thumbnail Fig. C.6

Spectral energy distributions of point sources in the Sh 201 field. We used the SED fitting-tool of ROB07.

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Eight point sources, identified in Fig. C.5, are detected at 100 μm around Sh 201. Five of these point sources have Spitzer counterparts in the catalogue of KOE08. Of these, four are Class I sources and one is Class II. Six of the point sources have 24 μm counterparts that are not included in KOE08 catalogue. We have measured the 24 μm fluxes of these six sources, as well as the Herschel fluxes. We give these data in Table C.1 (the values with an asterisk are our 24 μm measurements).

We fit the SEDs using the SED fitting-tool of ROB07, assuming an interstellar (external) visual extinction in the range 1.96 mag−5 mag; this last value is the mean extinction found for the whole associated cluster by Carpenter et al. (1993). We list the parameters of the YSOs, as well as the external extinction for the best models, in Table C.2. We show a few SEDs in Fig. C.6, and comment on individual sources below:

  • S201-a: this strong source is located in the direction of a brightfilament, south of Sh 201, and far from it. We showin Sect. 6.4 that, based on velocity arguments, thissource is not associated with the W5 region, but lies far in thebackground at about 5.45 kpc. (The parametersgiven in Table C.2 are obtained for this distance,allowing an external extinction free in the range1.95 mag to 10.0 mag, and nottaking into account the saturated 24 μm emission.) Twelve models satisfy χ2 − χ2(best) per data point  ≤3; all point to a massive Stage I YSO. Based on its relatively high temperature (Sect. 6.3.1) and luminosity we suggest that this source contains a small cluster, and is similar to BRC13-a or BRC14-c1+c2.

  • S201-b: this source is located to the south of Sh 201, in the direction of a low column density region. The Spitzer-IRAC images show a small group of stars in this direction; one is dominant at 8.0 μm and at longer wavelengths. It is a rather low mass Stage I YSO.

  • S201-c: the fit for this source is not very good, and only one model satisfies χ2 − χ2(best) per data point  ≤3. The model points to a Stage I YSO.

  • S201-d: this source is possibly located at the intersection of two filaments, to the south of Sh 201. This location is also a region of interaction between these filaments and the southern lobe of Sh 201. Many models satisfy χ2 − χ2(best) per data point  ≤3; they are all rather similar, but the fit is not good. S201-d is probably an intermediate mass Stage II YSO (the envelope accretion rate is low, null in some models), in agreement with KOE08. Figure C.5 shows an extended structure, jet-like, which seems to originate from the YSO (it is only observed at 100 μm and 160 μm); CO or OH lines, present in the PACS bands, and observed in regions of jets and shocks (van Kempen et al. 2010) could perhaps explain this feature. This feature, however, is not observed at 4.5 μm, a Spitzer band which traces shocks (Cyganowski et al. 2008, 2009).

  • S201-e: this source is located in the direction of the massive condensation on the east side, at the waist of Sh 201. Its SED is not constrained at all. The best model corresponds to a Stage I YSO, of intermediate mass. But numerous models satisfy χ2 − χ2(best) per data point  ≤3, which span a wide range of parameters. Thus, the nature of S201-e is very uncertain (from a low mass Stage I YSO to a high mass Stage II/III YSO).

The three other compact sources (f, g, h) have only Herschel measurements. Source f has a faint counterpart at all Spitzer wavelengths but it has not been measured by KOE08. Source g, which lacks a 24 μm counterpart and which is relatively bright at Herschel wavelengths, is probably a Class 0 source (Sect. 6.3.3). It is observed in the direction of the filaments bordering the massive condensation east of Sh 201. This location makes it a very good candidate for triggered star formation near the waist of Sh 201. We can estimate the mass of the g and h sources using the Herschel fluxes (Table 5); these masses are respectively  ~46 M and  ~4 M. We hypothesize that sources g and h are Class 0 sources.

thumbnail Fig. C.7

Young sources at the waist of Sh 201. Top: unsharp-mask 100 μm image of the eastern portion of the waist of Sh 201 showing the presence of two point sources. Middle: same but deeper image with the column density contours in green showing the dense condensation east of Sh 201. The column density contour levels are 1022, 3 × 1022, 4 × 1022, 5 × 1022, and 6 × 1022 cm-2); the red contours are for the 100 μm emission (levels of 0.5, 1.0, 1.5, and 2.0 Jy/beam). Bottom: the 100 μm contours are superimposed on the Spitzer 4.5 μm image showing the stars discussed in the text. The coordinates are J2000.

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The bright Spitzer-IRAC sources #1, #2, #3, and #4, located at the waist of Sh 201 and identified in Fig. C.2, have been classified as Class II sources by KOE08. According to Ojha et al. (2004), source #2 is the main exciting star of Sh 201, whereas sources #1 and #3 are early B stars. Source #4 is associated with bright extended emission at 24 μm, as is source #2; we suggest that it is also an early B star. Sources #5, #6, and #7 are Class I sources according to KOE08. They are bright 24 μm sources (no fluxes were given by KOE08; we measured these fluxes and give them with asterisks in Table C.1). None of these sources are detected at 100 μm. Their SEDs are not well-constrained. Most of the models for YSOs #5 and #7 indicate Stage II YSOS: their envelope accretion rates are low, even zero in some models, and their disks’ masses are high (#5 and #7 only). But the mass and age of the central objects are not at all constrained. Four other sources have been classified as Class I in the field of Fig. C.2, which have no 100 μm counterparts. They are #17667, #17531, #17526, #17620. Except for #17667, they also have very faint or no 24 μm counterparts. Thus, their classification as Class I is doubful.

Two other sources detected by Herschel, s1 and s2, lie at the eastern part of the waist of Sh 201, between the bright PAH-emitting filaments bordering the central ionized region and the massive molecular condensation. They are observed at 100 μm and 160 μm, but are not detected at shorter wavelengths and are not easily discernable at longer wavelengths, probably due to the lower resolution and the presence of the adjacent parental molecular condensation. In Fig. C.7 we show an unsharp-mask image at 100 μm of the field containing these sources. To create the unsharp-mask, we median filtered the PACS 100 μm image with a square window of 25″ and subtracted the filtered image from the original one. Figure C.7 (top) shows the two point sources, of which s1 is the brightest, and Fig. C.7 (middle) shows the 100 μm filaments in the PDR. The 100 μm flux of s1 and s2 are uncertain by a factor 2 because of their proximity to one another and because of the bright underlying filaments. s1 lies at α(2000) = 03h03m20.08s, δ(2000) =  + 60°28′01″ and has a 100 μm integrated flux  ~80 Jy. s2 lies at α(2000) = 03h03m19.35s, δ(2000) =  + 60°28′09″ and has a 100 μm integrated flux  ~36 Jy. Using the 24 μm detection limit  ~7 mag, we obtain a spectral index in the range 24 μm−100 μm α ≥ 7.2 for s1 and  ≥ 6.65 for s2. These very high indexes indicate that these sources are dominated by their envelopes. These sources are compact and their 100 μm fluxes are large. Thus, they are good candidates for high-mass Class 0 sources. Additionally, their location makes them good candidates for triggered star formation. An H2O maser has been detected at 22 GHz by Blair et al. (1980) in the direction of star #1 (Fig. C.7); however, due to the resolution of these observations (~1.4′), it is unclear if it is associated with #1, s1 or s2.

Appendix D: The region between W5-E and W5-W: W5-NE

Molecular material with a rather high column density of  ~5 × 1021 cm-2 is present between W5-E and W5-W (see Fig. 4). Two small H ii regions are located here, surrounded by dense material. A bright rim is also present, turned towards W5-E and its exciting star. Several point sources, listed in Table D.1, are detected at 100 μm and at longer wavelengths.

D.1. The H II regions

At least two H ii regions are present in the region located between W5-E and W5-W. In Fig. D.1 we show the Hα emission of the ionized gas. The exciting star of the A H ii region is unknown; we identify it with an optical star at the center of the optical and IR nebula that we call ex1 (see Figs. D.1 and D.2). Wilking et al. (1984) call this IR source IR2, and, using several indicators, propose a spectral type in the range B0–B2 ZAMS for the central star. The B H ii region is excited by BD+60596, a B1V star (Hiltner 1956) which lies at α(2000) = 02h57m08s02, δ(2000) =  + 60°39′44.′′1 (a few other spectral types have been proposed for this star: B3III, Garmany & Stencel 1992; B0.5V-III, Zdanavicius & Zdanavicius 2001).

The NVSS survey at 20-cm shows radio-continuum emission from three regions, the A and B H ii regions and the border of a bright rim (Fig. D.1; two components are listed in the NVSS catalogue for the B H ii region). Figure D.2 shows in green the Spitzer-IRAC 5.8 μm extended emission coming mostly from the PDR of the large W5-E and the PDRs surrounding the small H ii regions. Figure D.2 also shows in red the Spitzer-MIPS 24 μm extended emission due to the hot dust located inside the ionized regions; it allows to locate the exciting stars. A 24 μm extended emission appears north of the A H ii region. It probably points to the presence of another early B star (there is an extention of the radio contours in this direction). An optical and IR star, that we call ex2, lies at the center of this extended emission.

The three stars (ex1, ex2, and BD+60596) are in the catalogue of YSOs by KOE08. They are listed in Table D.1; ex2 and BD+60596 are classified as Class III by KOE08; ex1 is classified as Class II. This classification is uncertain as these stars are at the center of bright 24 μm extended emission. We can use the K magnitude and J − K colour to estimate (a very coarse estimate) the spectral type of these three stars. Using a distance of 2 kpc, the 2MASS photometry of ex1 indicates a B1–B2 spectral type with a visual extinction  ~3.4 mag. Using the NVSS flux density of 28.8 mJy for the A H ii region, we estimate a ionizing photon flux log    (NLyc) = 45.9 pointing to a spectral type B2V (Smith et al. 2002); if we consider the CGPS integrated flux of 0.13 Jy (Taylor et al. 2003) we obtain log    (NLyc) = 46.4 ionizing photons per second, corresponding to a B1V spectral type (Smith et al. 2002). These results are in agreement with Wilking et al. (1984) spectral type. The 2MASS photometry of ex2 indicates a B1–B2 spectral type but with a higher visual extinction  ~6.9 mag. The 2MASS photometry of BD+60596 indicates a spectral type in the range B0V–B1V, with a low visual extinction  ~1.4 mag. The integrated CGPS flux density of 0.31 Jy gives log    (NLyc) = 46.9 pointing to a B0.5V spectral type (Smith et al. 2002); these results are in good agreement with the previous determinations.

Table D.1

Photometry of point sources: the region between W5-E and W5-W.

D.2. The YSOs

Figure D.3 shows the Herschel-PACS 100 μm image of the field. The colour image allows us to compare the location of the hot dust emitting at 24 μm with the dust emitting at 100 μm. The bright extended 24 μm emission comes from the inside of the H ii regions and from the PDRs bordering them, whereas the extended 100 μm emission comes only from the PDRs. The A and B H ii regions are rather different. The B H ii region is bordered by a dense PDR on its east side and seems to open on the west side. The low extinction of its exciting star indicates that it lies in front of most of the material along this line of sight. The A H ii region is more compact, but its influence seems to extend over a large region. The extinction of its exciting star is larger than that of BD+60596, indicating that it is more embedded. A bright feature at 100 μm, bar-like, lies very close in projection to the ex1 exciting star.

thumbnail Fig. D.1

Ionized gas in the region between W5-E and W5-W. The NVSS radio-continuum contours at 20-cm (contour levels 0.002, 0.005, 0.0075, 0.01, and 0.015 Jy/beam) are superimposed on a grey-scale image of the Hα emission. The Hα image has been obtained at the 1.2-m telescope of the Observatoire de Haute Provence. Two H ii regions are present in this field, excited by the stars ex1 and BD+60596. Radio continuum emission is also observed at the border of a bright rim.

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thumbnail Fig. D.2

Colour composite image of the region located between W5-E and W5-W. Red is the Spitzer-MIPS emission from the hot dust at 24 μm, green is the Spitzer-IRAC emission at 5.8 μm from the YSOs and the PDRs (PAH emission), blue is the Hα emission of the ionized gas.

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thumbnail Fig. D.3

Colour composite image of the region between W5-E and W5-W. Left: red is the Herschel-PACS emission at 100 μm, and turquoise is the Spitzer-MIPS emission at 24 μm. Right: the point sources detected at 100 μm and discussed in the text are identified on a grey-scale image of the 100 μm emission (given in logarithmic units).

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thumbnail Fig. D.4

Features discussed in the text. Left: colour composite image of a small bubble; red is the 24 μm image, turquoise is the Spitzer 8.0 μm image showing the PAH emission. Middle: YSO IB-e and its vicinity; red is the 100 μm emission, turquoise is 8.0 μm emission. Right: YSO-a; the underlying grey image is the 100 μm emission, the red contour shows the position of the 24 μm Class I source a1.

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Table D.2

Parameters of the YSOs in the region between W5-E and W5-W, obtained with the SED fitting tool of ROB07.

thumbnail Fig. D.5

Spectral energy distributions of point sources in the region between W5-E and W5-W. We used the SED fitting tool of ROB07.

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Seven 100 μm point sources are present in this field, identified in Fig. D.3, and listed in Table D.1. We fit some SEDs using the SED fitting-tool of ROB07 (assuming an external visual extinction of 1.96 mag). The obtained parameters are given in Table D.2. We comment on individual point sources below (the name IB refers to “in between”).

  • IB-a: this source is the brightest of a group of possibly three100 μm sources. A detailed view of this region is given in Fig. D.4. It has a very faint 24 μm counterpart, and thus is a candidate Class 0 source. Using only the Herschel fluxes we obtain for the envelope of this source a dust temperature of 13.7 K and a mass of 16.5 M (Table 5). The Class I YSO a1 in KOE08 catalogue has a faint 100 μm counterpart, too faint to be measured accurately. The western 100 μm source has no 24 μm counterpart at all; it is also a candidate Class 0 YSO.

  • IB-b: this source is a Class I YSO according to KOE08. We show its SED in Fig. D.5. Three models satisfy χ2 − χ2(best) per data point  ≤3. The SED of this source points to a Stage I YSO, with a central object of about one solar mass, an envelope of 8.7 M (Table 5), and a rather high accretion rate.

  • IB-c: this source is a Class II YSO according to KOE08; however a FIR counterpart appears to be present on all Herschel images. The SED is not well fit as shown by Fig. D.5; however it points to a Stage I YSO. We cannot exclude the existence of two distinct sources: a Class II YSO adjacent to a condensation of 8.5 M (Table 5).

  • IB-d: this source is a Class I according to KOE08. We agree with this classification, but again the SED fit is not good. The parameters of the model give a Stage I source with a massive envelope (20.3 M; Table 5), and a high envelope accretion rate. We did try the fits with the extinction as a free parameter; the fit is better (according to χ2) with a higher external extinction (10.38 mag), but the fitted model is very similar to the previous ones.

  • IB-e: this 100 μm source is slightly elongated in the east-west direction. It is a complicated region as a small group of about half a dozen stars lies in its direction (see Fig. D.4). Only three YSOs (e1, e2, and e3) are in KOE08 catalogue. None of them are associated with the 100 μm source, as shown by Fig. D.4. (The 24 μm source resembles the 100 μm source; it is elongated east-west.) Two sources appear at 8.0 μm in the direction of IB-e (which probably explains its elongation); they are not present in the catalogue of KOE08. Source e is probably associated with these two sources, which thus probably are Class I YSOs.

  • IB-f: this source has been identified as a Class II YSO by KOE08. Two other faint stars lie nearby, but f is dominant at all Spitzer wavelengths. The SED is not well fitted by the SED fitting-tool of ROB07, as shown by Fig. D.5. This source is V LW Cas, a variable star of uncertain nature (Kukarkin et al. 1971), not a cataclysmic variable (Downes & Shara 1993), not a symbiotic star (Henden & Munari 2008). It is probably evolved, and we do not know if it is associated or not with W5-E.

  • IB-g: this source is a Class II YSO according to KOE08. It has a 100 μm counterpart, but nothing is detected at longer wavelengths. Numerous models have an acceptable fit to the SED. The model parameters point to an intermediate mass YSO, probably in Stage I; but the parameters of the disk are not well contrained.

The field of Fig. D.3 contains six other Class I YSOs according to KOE08; they are identified in this figure. YSOs a, a1, b, #1, #2, and #3 are part of a high column density ridge extending east-west; ex2 is possibly part of this ridge. This structure therefore appears to be an active star forming region, but its origin is unclear. YSOs #1 and #2 have a very faint counterpart at 100 μm (too faint to be measured). YSO #4 is observed in the direction of the B H ii region; it has a very faint counterpart at 100 μm. YSO #5 has no 100 μm counterpart. It is faint at 24 μm and therefore its classification by KOE08 as a Class I is uncertain. YSO #6 is peculiar. It has an extended and bright 100 μm counterpart, located on the border of a bright condensation. A close examination of the Spitzer images shows that this source is also extended at all Spitzer wavelengths. This condensation, centered at α(2000) = 02h57m33s02, δ(2000) =  + 60°27′32.′′ (position of the peak column density, 6.1 × 1021 cm-2) has an integrated flux at 250 μm  ~ 10.5 Jy in an aperture of radius 20″. The temperature map indicates a mean temperature  ~23 K for this extended structure and we estimate its mass to be  ~6 M.

Another peculiar structure is present in the field. It is a small bubble (diameter  ~35″, thus  ~0.3 pc), well traced by the PAH emission at 8.0 μm (see Fig. D.4). It contains three YSOs, classified as Class II by KOE08. One of them is especially bright at 24 μm and possibly extended; it has no 100 μm counterpart. We propose that this source, identified as s1 in Table D.1 and Fig. D.4, is a B star heating the nearby dust. (Its K magnitude and J − K colour could correspond to a B5–B7 star with an extinction AV ~ 5.6 mag.)

Appendix E: Isolated point sources

Table E.1

Photometry of isolated point sources detected at 100 μm.

Table E.2

Parameters of the isolated YSOs, obtained with the SED fitting tool of ROB07.

thumbnail Fig. E.1

Colour composite images of three isolated sources discussed in the text. Red is the Herschel 100 μm emission and turquoise is for Spitzer 8.0 μm emission (isolated 5 and 11) or the 24 μm emission (isolated 14bis). The grey inserts show, at the same scale, the 100 μm source.

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There are some 100 μm point sources in the W5-E field which lie outside the regions discussed previously. We give their measured Herschel fluxes in Table E.1. Four of these sources lie outside the coverage of the Spitzer images and therefore they have no measured Spitzer fluxes. Four other sources have faint Spitzer counterparts that have not been measured by KOE08. Another source (#4) is isolated, bright at 8 μm and 24 μm, but curiously, is absent in the KOE08 catalogue. When possible we have meaured the 24 μm fluxes (these measurements have an asterisk in Table E.1). We attempt to classify them, using the spectral index of the SED between 24 μm and 100 μm (definition in Sect. 5). According to this metric, sources #1, #4, #5 (in agreement with KOE08 classification), #6, #7, #8, and #9 are Class I sources., sources #11 is a flat spectrum source, and source #12 is a Class II source (again in agreement with KOE08).

When possible we used the SED fitting tool of ROB07, assuming an interstellar visual extinction of 1.96 mag. The results are the following (see also Table E.2):

  • Isolated-5: this source lies at the end of a small globule, not farfrom an IF and its associated PDR(Fig. E.1). It is possibly the head of a pillarseparated from the PDR (similar to what isobserved in the region of the pillars to the south-eastof W5-E; see Sect. 6.2). Theemission we detect at 100 μm may also be the counterpart to the Class I YSO identified by KOE08. Five models satisfy χ2 − χ2 (best) per data point  ≤3, all of which point to a low-mass Stage I source. But we cannot exclude that we are dealing with two distinct sources, a Class I YSO close to or inside a globule. Using only the Herschel fluxes and the modified blackbody model we estimate a mass of 1.7 M for the globule’s head or the YSO’s envelope (Table 5).

  • Isolated-6: as shown by Fig. E.2 the SED is not well-constrained. A large range of masses and ages are possible for the central source. However, the fact that the envelope accretion rate is never zero, possibly disagrees with KOE08 classification as a Class II.

    thumbnail Fig. E.2

    Spectral energy distributions of a few isolated point sources. We used the SED fitting-tool of ROB07. (In isolated-11 we use only the flux at 100 μm.)

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  • Isolated-11: according to KOE08 this source is a Class I YSO. It is a bright Spitzer and Herschel source, point-like at 100 μm (as shown by Fig. E.1). Only one model satisfies χ2 − χ2(best) per data point  ≤3 for the whole range of wavelengths. Its parameters, given in Table E.2, correspond to an intermediate mass Stage II/III source. Figure E.2, however, shows that the fit is not satisfactory. When we use only the 100 μm flux the fit is much better (SED of isolated-11). In this case the SED corresponds to a Stage I YSO of about 1 M. As shown by Fig. E.1, the source lies near an IF, and is surrounded by a small bright rim. One possible explanation is that, at Herschel-SPIRE wavelengths we see the emission of the cold condensation enclosed by the bright rim, whereas at Spitzer and at 100 μm we see the Class I YSO, possibly a Stage I YSO recently formed inside this condensation. Better spatial resolution in the FIR is necessary to confirm this interpretation.

  • Isolated-12: several very similar models satisfy χ2 − χ2(best) per data point  ≤3 and therefore we give only the best one in Table E.2. All point to a Stage II source, in agreement with KOE08 classification. This is an intermediate mass YSO, with no accreting envelope.

  • Isolated-14bis: this source lies in the direction of PDR 2. It is classified as a Class II YSO by KOE08. It is bright at 24 μm but its 100 μm counterpart is rather faint (Fig. E.1). A Class I YSO lies nearby that we call isolated-14 (isolated-14 is very faint at 24 μm and has no 100 μm counterpart; thus, its classification is uncertain). For isolated-14bis several models satisfy χ2 − χ2(best) per data point  ≤3, which span a large range of parameters. The classification of isolated-14bis is therefore uncertain. A better fit is obtained if we let free the external extinction (up to 10 mag). This source is clearly a Stage II YSO (of intermediate mass), as all the models give an envelope accretion rate equal to zero and a massive disk.


© ESO, 2012

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