High-resolution CARMA Observation of Molecular Gas in the North America and Pelican Nebulae

CARMA observations were carried out in 2017 during the last season of array operations. The instrument setup was the same as that adopted for the CARMA–NRO Orion Survey (Kong et al. 2018, hereafter K18). Briefly, the 15-element array of six 10 m diameter and nine 6 m antennae was used to map the area of interest. The total mosaicked area of about 1 deg² is made up of 126 pointings of 6′ × 6′ subfields. The “D”’ and “E” configurations of the array led to uv-coverage between 2.5 and 40 k λ, where λ is 2.6 mm for ¹²CO(1-0), resulting in an angular resolution of about 7″ (see Table 1).

As for the Orion Survey, the spectral line setup focused on ¹²CO(1-0), ¹³CO(1-0), C¹⁸O(1-0), but also included the CN N = 1-0, J = 3/2-1/2, SO 2(3)-1(2), and CS(2-1) lines. Here we report only on the ¹²CO(1-0), ¹³CO(1-0), and C¹⁸O(1-0) observations. With regard to ¹²CO, the correlator setup provided a bandwidth of 31 MHz (81 km s⁻¹), and a spectral resolution of 98 kHz (∼0.25 km s⁻¹). For the other spectral lines, a bandwidth of 8 MHz (∼21 km s⁻¹) with spectral resolution 24 kHz (∼0.067 km s⁻¹) was used.

To compensate for the fact that the interferometer observations did not include baselines shorter than 2.5 k λ, effectively filtering out emissions from structures larger than 100″, the CARMA observations were combined with single-dish maps of the same regions. These were obtained using the Purple Mountain Observatory Delingha 13.7 m telescope (hereafter DLH14) (Zhang et al. 2014, hereafter Z14), and are part of the Milky Way Imaging Scroll Painting project (Su et al. 2019).

The CARMA and DLH14 data were combined in the uv plane, following the procedures outlined in Koda et al. (2011), and Kong et al. (2018). The resulting images are based on observations ranging from zero-spacing fluxes to measurements on the maximum baselines provided by CARMA. Figure 2 shows a comparison of CARMA and DLH14 uv sensitivities. The uv-plane pixel sensitivity as a function of baseline length was calculated as per Appendix C in Koda et al. (2011). It is a function of the image-plane sensitivity and the dish size or, for an array, baseline length.

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In general, to ensure good results in terms of imaging quality when combining observations from two instruments, it is desirable that the sensitivities match where the baselines overlap. Figure 2 shows that this is not the case here; the sensitivity of the DLH14 data is much lower that that of CARMA. The sensitivity mismatch also affects flux-scale determination adversely. As described in Kong et al. (2018), the relative fluxes of the single-dish and CARMA data can be calibrated based on the flux ratio within the overlapping baselines. Here, the DLH14 data is sufficiently noisy, beyond baselines of 2 k λ, that no meaningful flux-scale factor can be established (we therefore adopt the factor of 1). Nevertheless, as we demonstrate below, the DLH14 observations make a useful contribution to our final images, in spite of their quantitative issues. Table 1 summarizes the final sensitivity for the combined data. To match the single-dish observations, a spectral resolution of 0.16 km s⁻¹ was adopted for the combined ¹³CO and C¹⁸O data set.

To allay concerns about the reliability of the NAP images that result from combining CARMA and DLH14 observations, we compare ¹²CO integrated-intensity images of the Pelican Head region from DLH14, CARMA, and combined CARMA+DHL14 data. The Pelican Head area was selected, since it appears to encompass multi-scale structures (see Figure 3). The DLH14 map (left panel) shows smooth, extended emission with very little structure, while the CARMA map (middle panel) is irregular, showing the distinct (sharp) edges of the cometary-like cloud in this region. The combined image (right panel) clearly displays the usefulness of adding short-spacing flux information; considerably more of the structure appears to be present, both at the edges and in the body of the cloud.

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In line with the CARMA–NRO Orion Survey (K18), we carry out a Δ-variance analysis (Stutzki et al. 1998; Ossenkopf et al. 2008) for the channel maps and integrated-intensity maps, so as to examine the quality of the data combination. All main features are visible in the integrated-intensity maps. Figure 4 shows the Δ-variance spectra for the Pelican Head (left panel), and the Gulf of Mexico (right panel) regions. In both regions, and for all tracers, there is a structural break at a scale of 1′–2′, corresponding to the gap in our spatial sensitivity at 1.7–3.4kλ, due to the lack of sensitivity at these scales (see Figure 2). The dip in the Δ-variance at that scale also reflects the mismatch of sensitivity in the overlapping baseline lengths.

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Figure 5 shows a composite image of the Gulf of Mexico and Pelican regions (the same area shown in Figure 1) compiled using Spitzer data at 24 μm (red), 8 μm (green), and 4.5 μm (blue). Figure A1 shows a Wide-field Infrared Survey Explorer (WISE) three-color composite image of the same region, in which we assigned red for the 22 μm emission, green for the 12 μm emission, and blue for the 4.6 μm emission. As in Figure 1, We indicate the extent of the CARMA maps with white boundaries.

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In both Spitzer and WISE images, the Gulf region shows a few relatively bright-rim structures. These bright rims trace hot dust in the area. For instance, the WISE W4 band (22 μm) traces the warm dust thermal radiation. Meanwhile, the WISE W3 band (12 μm) includes many emission features from PAH (at 7.7 μm, 8.5 μm, 10 μm, and 11.3 μm), and fine-structure lines from [Ne ii] (12.8 μm) and [Ne iii] (15.6 μm). These lines are probably pumped by UV emission (Allamandola et al. 1989), and are believed to be good indicators of star formation and stellar feedback (e.g., Ho & Keto 2007). In any case, the hot dust should be heated by a nearby ionizing source.

Here, we argue that the ionizing source responsible for the bright rims seen in these IR images is the Bajamar Star, simply due to the proximity of this high-mass, UV-irradiating young star to these features. As we show in Section 3.2, the labeled bright rims inside the CARMA map area have clear molecular line counterparts. They provide the key connection between molecular gas and massive-star feedback. Hereafter, we designate the bright-rim filamentary structures west and south of the Bajamar Star as Rim1, Rim2, and Rim3, and the bright-rim structures with a boomerang-like morphology to the southeast of the Bajamar Star as Boomerang1 and Boomerang2. There are a few more bright-rim structures in the infrared images which we do not label, as they are either less prominent, or do not have molecular line counterparts.

The morphology of the bright-rim structures can also be used to deduce that the Bajamar Star is the most likely source of feedback in this region. A line passing through the centers of the two boomerang structures and extending to the northwest also passes through the position of the Bajamar Star (see the cyan arrow in Figures 1 and 5). There are two more arc-like features to the northeast of Boomerang2. These features, which are not as bright as Boomerang2, point toward the general direction of the Bajamar Star (see Figures 1 and 5).

As stated above, there is evidence that the Bajamar Star is the main ionizing source in the Gulf region. Straižys & Laugalys (2008, hereafter SL08) reported 12 additional massive-star candidates in the NAP. We determined the distances to these massive stars (including the Bajamar Star) based on the Gaia DR2 results (Bailer-Jones et al. 2018). The Bajamar Star (star #8 in SL08) has a distance of pc. However, updated results based on Gaia EDR3 find a distance to the Bajamar’s star of d = 785 ± 16 pc, in excellent agreement with the 3D dust and YSO-based results discussed in Section 4.3 of this work. The IRS4 source (star #2 in SL08) in the Pelican region (Bally & Scoville 1980) has a distance of pc. However, SL08 suspected that IRS4 is a carbon star. All other candidates are beyond 1500 pc, and there are two sources with no Gaia distances (star #11 in SL08, a.k.a, 2MASS J20580673+4355141; and star #12 in SL08, a.k.a. 2MASS J20582424+4356386). As shown in Figure 1 of B14, these two sources are far to the east of the Bajamar Star. Given the alignment between the Bajamar Star and the bright-rim features discussed above, the Bajamar Star is the only likely massive star responsible for these feedback features in the Gulf region.

We further argue that the Bajamar Star is also the main ionizing star impacting the Pelican region, as suggested by Damiani et al. (2017). The Pelican Head region is clearly being impacted by UV radiation. In Figures 5 and A1 we notice that the Pelican Head region has very bright infrared emission (mostly notable at λ ≳ 5 μm, i.e., visible in IRAC band 3 at 5.7 μm, but barely noticeable in WISE band 2 at 4.6 μm). The same region also exhibits plenty of H₂ fluorescent emission, caused by UV emission (as reported by B14), which matches well with the infrared filaments here. In addition, as we discuss in Section 3.2, there are two bright emission features with a comet-like (i.e., head-tail) morphology in the Pelican Head region which were very likely caused by feedback from a massive ionizing star, and which point in the general direction of the Bajamar Star. Hereafter, we therefore assume that all feedback features in this area are the result of feedback from the Bajamar Star.

As can be seen in the ¹²CO and ¹³CO 0th-moment maps of the Gulf region (Figures 6(d) and (e)), molecular gas is present over practically the entire area. The only notable decrease in emission occurs toward the southeast corner (near the Boomerang1 structure), and toward the northern-central edge of the map. There are also a number of bright prominent regions throughout the maps. These can be seen more clearly in the peak-intensity maps shown in Figures 6(a) and (b), and discussed below. In contrast, the C¹⁸O emission is concentrated in the central-eastern part of the map (Figures 6(c) and (f)). This emission, as discussed below, coincides with the denser parts of the cloud, where 1.1 mm dust-continuum emission has been detected.

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Figures 6(a)–(c) show the peak-intensity maps of ¹²CO(1-0), ¹³CO(1-0), and C¹⁸O(1-0) for the Gulf region. We overlay the Bolocam Galactic Plane Survey (BGPS) 1.1 mm continuum emission (Ginsburg et al. 2013), denoted by white contours. In the Gulf region, the figures show a group of six prominent continuum sources toward the east side of the maps. These are all part of the group of cores traced in 1.1 mm continuum emission, and labeled as “Gulf total” by B14. This includes the two cores designated “Core E” and “Core W” in the northeast part of the map. South of Core W lies the “SW1” core, and toward the southwest of SW1 (from north to south) lie the “SW2 main,” “SW2 SE,” and “SW2 S” cores (B14). These latter three (i.e., the SW2 cores) have prominent C¹⁸O counterparts (Figure 6(c)).

There are various prominent features of bright molecular line emission in the Gulf region. Between continuum cores W and SW2 main lies a region of high ¹²CO and ¹³CO emission, primarily extending from east to west, which we call the “Wall” (see Figure 6(a) and (b)). It is likely that the Wall, which is at least twice as bright as the average emission in the rest of the map, traces the warm gas heated by a young star lying just north of this structure, which is also responsible for the bright IR nebulosity (hereafter SW-IR) seen in Figures 5 and A1.

Additional prominent line emission regions can be seen in the southeast corner and the mid-western part of the map. In the southeast, the bright emission is coincident with the Boomerang1 feature discussed above (see Section 3.1 and Figure 5). In the central-western region of the map, the bright emission is associated with the IR features labeled Rim1, Rim2, and Rim3 (see Figure 5 and Section 3.1). These, in general, show a sharp increase in intensity toward the edge of the emission structure facing the Bajamar Star (aligning with the IR bright rims defined in Section 3.1 and Figure 5), and have a chaotic morphology in ¹²CO (Figure 6(a)) and ¹³CO (Figure 6(b)) away from these bright rims. As discussed later in this paper, these structures are part of the filament F-1, as defined in the study by Z14.

In Figure 7 we show a three-color image comparing the WISE 12 μm band, the peak-intensity map of ¹³CO, and the peak-intensity map of ¹²CO. It is very clear that the bright-rim features in the IR image are associated with the bright filaments in molecular line emission. Rim3 overlaps with ripple-like features in CO emission. Moreover, the entire molecular gas structure connected with the rims appears as shredded pieces. A plausible explanation for this is that the expanding bubble has reached (or is very close to) the molecular gas associated with these structures, and the gas therefore exhibits this chaotic morphology because it is being dispersed by the UV photons from the massive star.

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In the Pelican Head region, we detect ¹²CO (Figures 8(a) and (d)) throughout most of the mapped area, following a morphology roughly similar to the structure delineated by the yellow-colored features in the Spitzer and WISE three-color composite images. The ¹²CO, ¹³CO, and C¹⁸O integrated-intensity maps (Figures 8(d)–(f)) show three separate regions of relatively high-intensity emission: in the north (around decl. 4458), center (around decl. 4443), and south (around decl. 443), respectively. These structures are also clearly shown in the peak-intensity maps (Figures 8(a)–(c)). The northern and southern high-intensity regions (which we name Comet1 and Comet2, respectively) have comet-like structures, with bright narrow heads, and with tails extending to the northwest. The central high-intensity structure (which we name Rim4) has a wide arc-like edge and extended (bright) emission to the west, somewhat similar to the Rim2 structure in the Gulf region. All these features roughly point toward the Bajamar Star, and are thus very likely caused by feedback from the massive star.

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Comet1 is shown in detail in Figure 3. The CARMA high-resolution data shows some ripples at the boundaries of Comet1 (particularly at the northern boundary). Interestingly, both cometary clouds have BGPS sources at their head location, reminiscent of triggered star formation by radiatively driven implosion (Bertoldi 1989). The northern one corresponds to the “Pelican MHO 3402” core in B14 (Figure 3, Table 4), while the southern one corresponds to the “Pelican 8” core in B14.

Figures 9(a)–(c) show intensity-weighted average velocity (1st-moment) maps of the Gulf region. These provide a general picture of the complex kinematics in this region. In the ¹²CO and ¹³CO 1st-moment maps (Figures 9(a) and (b)), the molecular gas associated with the IR bright rims (i.e., Rim1, Rim2) can be seen as gas with average velocities of about −5 km s⁻¹, in sharp contrast with adjacent gas at more redshifted velocities.

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In contrast to the Gulf region, the Pelican Head region (Figures 10(a)–(c)) shows a significantly narrower range of intensity-weighted average velocities. Here, most of the emission is concentrated at velocities between −2 and 2 km s⁻¹. In the ¹³CO 1st-moment map (Figure 10(b)), Comet1 is discernible as an oval region with velocities around 2 km s⁻¹. Comet2 and Rim4 structures are not as conspicuous as Comet1, but are similar to the other feedback features in the NAP, in that the gas beyond the sharp edge is at redshifted velocities, as compared to the feedback structure.

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Figures 9(d)–(f) show 2nd-moment maps of the Gulf region. In the ¹²CO and ¹³CO maps (Figures 9(d) and (e)), the gas associated with the bright rims shows large velocity dispersion. On the other hand, the C¹⁸O gas (Figure 9(f)) has narrow velocity widths, in sharp contrast with the ¹²CO and ¹³CO gas. Figures 10(d)–(f) show 2nd-moment maps of the Pelican region. The gas facing the Bajamar Star exhibits larger velocity dispersion. The gas behind this front is more quiescent. Interestingly, Comet1 does not show large dispersion in either ¹²CO or ¹³CO maps.

The moment maps by themselves are not sufficient to permit differentiation between the different cloud components. The distribution of dark patches in the optical image of the region, in conjunction with the location and kinematics of the molecular gas, can be used to derive an approximate three-dimensional distribution of the gaseous structures observed in our map.

The dark patches in the optical image of the NAP in Figure 1, particularly those in the Gulf and Atlantic regions, have generally been thought to be caused by molecular clouds in front of the H ii region, mostly at the same distance (B14, Z14). However, a closer look at Figure 1 suggests a more complicated story. The dark clouds in this image show at least two shades of darkness. One is a semi-transparent gray silhouette, running from the top-right (northwest) edge of the figure throughout the Atlantic area, extending south of the Bajamar Star.

A large area southeast of the Bajamar Star also shows this semi-transparent grayish hue. We designate areas with this shade as “gray regions.” In Figure 1, we also see a collection of patchy clumps that are much more opaque than the gray regions, and which traverse from east to west across the entire Gulf region. We name these areas “dark regions.” These two different shades, combined with information from images at other wavelengths (together with the gas kinematics), imply that there may be more than one group of molecular gases (possibly at different distances) along the line of sight, consistent with our inferences based on the kinematics information. Determining the relation between these two groups of clouds is important in terms of identifying the molecular gas responsible for the star formation taking place in this region, and of understanding the impact of feedback on the molecular clouds in the NAP.

In Appendix B and Figures B1–B7, we identify distinctive, coherent features in the ¹³CO position–position–velocity (PPV) cube, and compare the gas distribution with different features depicted in the POSS-II red color plate. Figure 11 summarizes the major structures defined, based on this comparison.

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Figure 12 shows a schematic of the line-of-sight structure of the NAP complex. The locations are given in a relative sense, and are based on the findings discussed in Section 3.4 and Appendix B. We include the major structures defined in Appendix B. These structures occur mainly in the Gulf and Atlantic regions, with a few in the Pelican region and behind the W80 bubble.

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There are three major groups of clumps and filaments, based on their line-of-sight distance. The closest to us comprises a group of dark clumps of varying morphologies (see Figure 11 and the rightmost dark structures in Figure 12). These are in front of the W80 bubble, and appear as dark shadows in the POSS-II images. The group members are small and clumpy, with many ∼10,000 au globules. Their V_lsr is similar to that of the bubble.

Between the dark group and the W80 bubble is the group of gray clouds. F-1 is very close to the bubble, being part of this group; it is lit in the mid-to-far-infrared, and shows bright rims (Section 3.4 and Figures 5, A1, 6, B1). F-2, F-3, and F-5 are also close to the bubble, particularly in the Atlantic region, based on their similar extinction and velocity to the F-1 filament. F-6 may not be so close to the bubble as F-1, but it is between the bubble and the Dark Crab. Boomerang1 and Boomerang4 appear to be dark in the optical image. They are located on the near side of the bubble. Boomerang2 and Boomerang3 are probably on the edge of the expanding bubble.

Behind the bubble are several background clumps, including F-4 and F-7. These do not show gray/dark counterparts in the optical images, and are moving away at ∼5 km s⁻¹. It is not clear whether they are close to the bubble. We suspect that F-7 (covered by our CARMA observations) is not very close to the H ii region, as it does not exhibit the “shredded” and chaotic structure of F-1.

As an independent approach to associate the different velocity components to structures seen in the optical image as foreground layers, we correlate the ¹³CO data with the foreground extinction. We focus on the combined data in the Gulf region. Instead of using existing extinction maps that trace the whole column of material (e.g., Cambrésy et al. 2002), we obtain an approximate foreground extinction map from the POSS-II red image in Figure 11 by assuming that the highest intensity in the region represents the typical radiation from the bubble, and translating all lower intensities into an an effective red extinction. The resulting foreground extinction map is shown in the top left panel of Figure 13.

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We investigate the correlation between ¹³CO gas and extinction by computing the covariance between the optical image and the ¹³CO channel maps. ¹⁹ For the ¹³CO channel covariance maps (shown in 13), only noise contributions above 4σ were considered.

We find a strong correlation around −5 km s⁻¹ (the channel including F-1, Figure B1), −2 km s⁻¹ (including F-2, Figure B2), and 4 km s⁻¹ (including SW-Clump, Dark Filigree 1, Dark Heron, Figure B5). At 1.3 km s⁻¹ (including F-6, Figure B4) and 6.4 km s⁻¹ (including Dark Crab, F-7, Figure B7), we actually find a significantly weaker correlation, due to additional emission at those velocities, stemming from the background. Most importantly, the calculation suggests that the Dark Crab is a itself a structure, distinct from the F-6 filament. Although it appears at the same velocity as the F-7 gas, both its dark appearance at near-infrared wavelengths and its well-resolved structure indicate that it must represent material in front of the bubble, closer to us. This is important in terms of being able to match known YSO clusters in the NAP region with their host molecular gas (see Section 5.1). Of particular interest is the SW-Clump, which seems to have counterparts at −2 and 4 km s⁻¹. In the molecular line cubes of ¹²CO and ¹³CO, we find that this clump actually spans a wide velocity range, encompassing the velocity of the F-6 filament. The average spectrum of the SW-Clump shows two peaks at about −2 and 4 km s⁻¹. A possible explanation for this is that the clump is elongated along the line of sight, with its far side being pushed by the bubble to produce the negative velocity component. The SW-Clump is likely to be connected or associated with the F-6 filament.

In this section, we discuss the distance to the NAP based on YSOs, 3D dust mapping, and the presumed ionizing star (Bajamar’s star).

We begin by utilizing the Gaia distances (Bailer-Jones et al. 2018) of YSOs associated with dark clouds in this region to evaluate the distance to the NAP region. Figure 14 shows an overview of the YSOs in the NAP region from R11. This YSO classification was based on the near- to mid-IR spectral slope (from 2 to 24 μm, see Section 5.2 of R11). ²⁰ We cross-matched each of the 2000-plus sources in the catalog of NAP YSOs produced by R11 with the Gaia distance catalog (Bailer-Jones et al. 2018), using a search radius of 2″, finding that just over 1000 sources have Gaia measurements.

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In Figure 15, we show a histogram of Gaia distances for all the R11 YSOs available in the Gaia catalog. We zoom in to a range of distances between 0 and 1500 pc, as sources beyond 1500 pc are highly unlikely to be associated with the NAP complex. As shown in the histogram, there is clearly a peak around 800 pc (there is a tail extending from 1500 pc to about 7000 pc, but no other peaks). Assuming that the YSOs are close to, or associated with, the molecular gas in the NAP complex, then the histogram implies that these clouds are located at a probable distance of ∼800 pc. Recently, Zucker et al. (2020) reported a similar distance in regard to the NAP region, based on 3D dust mapping. Kuhn et al. (2020) also reached a similar conclusion, based on a more detailed study of stellar components of the NAP region.

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A distance of 800 pc, estimated using the YSOs, is at odds with the original distance from pc that we derived to Bajamar’s star, based on Gaia DR2 parallax measurements. Even if one considers the upper bound of the uncertainty, the deduced distance would be about 100 pc closer than the distance derived from either YSOs or 3D dust mapping. Fortunately, the higher-precision astrometry and improved treatment of systematic effects available with Gaia EDR3 has resolved the discrepancy in the previous DR2 parallax measurement; Kuhn & Hillenbrand (2020) report a new distance to the Bajamar’s star of 785 ± 16 pc.

In panels (b)–(d) of Figure 15, we separately plot the distance histogram for the three clusters defined in R11. Their names are Gulf, Pelican, and PelHat, and we mark their locations with white dashed ellipses in Figure 14. One can see from Figure 15 that all three YSO clusters peak at ∼800 pc. The PelHat cluster has a limited number of members, so its distance is uncertain. However, the Gulf cluster, which is associated with the foreground gray/dark features (Dark Crab and F-6, see Section 5.1), also has a peak distance of ∼800 pc. Since the W80 region is behind the gray/dark features, the distance to the W80 region must be farther than the Gulf cluster. While this contradicts the Gaia DR2 distance measurement for the Bajamar Star, it is entirely consistent with the more accurate Gaia EDR3 measurement (see Kuhn & Hillenbrand 2020).

We have produced an interactive distance map of the NAP, based on 3D dust mapping (available here), following the technique outlined in Zucker et al. (2019, 2020). Figure 16 shows the distribution of dust along the line of sight toward three representative subregions (≈1 deg² in area) of the map, centered at (l = 840, b = −18), (l = 844, b = 03), and (l = 851, b = −03), respectively. Similar region-by-region plots over the full cloud are available online. The distance uncertainties for the nebula do not include any systematic uncertainties, which we estimate to be 5% in distance.

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For the full map, see https://faun.rc.fas.harvard.edu/czucker/Paper_Figures/NorthAmerica.html. To produce the map, we grid the NAP region into individual pixels (based on the Healpix system, with a resolution parameter (N_side) of 64, Gorski et al. 2005) following Zucker et al. (2019), where each pixel covers about 1 deg² on the Sky. Each pixel is color-coded according to the distance we infer for the dust in that subregion. We are able to target all areas with an extinction of at least 5 mag. Each subregion is fit independently, with a typical uncertainty of about 5%–6% in distance, or about 40 pc. The distances obtained via 3D dust mapping are complementary to YSO-based distances, obtained either from Gaia DR2, or from VLBI observations (e.g., toward masers; Reid et al. 2019). Zucker et al. (2020) find that their 3D dust-mapping approach agrees with maser distances to within 10% out to 2.5 kpc, with no systematic offset.

While it is difficult to definitively constrain distance gradients within the NAP given our typical systematic uncertainty (≈40 pc), we determine that the entire NAP region traced by dust is consistent with its being at the same distance, to within a few tens of parsecs. Using distance information across the full cloud, we find an average distance of 797 ± 40 pc, in excellent agreement with the Gaia DR2 distances toward YSOs given in Figure 15. Our results are also consistent with the updated distance to Bajamar’s star—the presumed ionizing star—based on recent Gaia EDR3 results (from Kuhn & Hillenbrand 2020), who find that d = 785 ± 16 pc.

On a side note, the Gulf cluster distance histogram in Figure 15 shows a tentative excess at ≲800 pc, while the Pelican cluster shows a clearer excess at ≳800 pc. This distance difference implies that the Gulf cluster has members that are closer to us, which is consistent with the Gulf cluster being associated with a near-side clump/filament. Indeed, Kuhn et al. (2020) reported that stars in the Gulf region (their group E) are ∼35 pc closer than stars in the Atlantic and Pelican regions (their groups A to D at ∼795 pc).

Out of the three clusters discussed in Section 4.3, and shown in Figure 14, the Pelican cluster does not have a morphological match with molecular gas structures. The Pelican Head region (northwest of the Pelican cluster from R11, and south of the PelHat cluster from R11) has multiple MHOs, as identified in B14, indicating that the young stars there are still accreting from the molecular gas. However, the rest of the Pelican cluster (outside the Pelican Head region) has very limited ¹³CO(1-0) or C¹⁸O(1-0) emission, except for those young stars near the Atlantic region. R11 also noted that the cluster is in a low-extinction region (based on the extinction map of Cambrésy et al. 2002).

Kuhn et al. (2020) noticed that the Pelican cluster (their Group D) shows evidence of expansion. As discussed in Section 3.4, the Atlantic clouds are clearly associated with the bright rims, and are probably being dispersed by the Bajamar Star. A possible scenario is that the molecular gas associated with the Pelican cluster, probably the gas in the Atlantic region, is being dispersed by the massive star. The forming cluster was originally confined by the cloud’s gravitational potential, but is now expanding due to the (sudden) dispersal of the cloud. If true, then the Pelican cluster is a good example of cluster formation being halted by feedback from a nearby massive star. In other words, the cluster formation reaches an end, not because its associated gas is consumed or dispersed by the cluster itself, but by a neighboring massive star.

Here, we carry out a simple analysis to test the possibility that the cloud (hereafter the pre-Pelican cloud) in which the Pelican cluster formed might be being dispersed by the Bajamar Star. We note that the PelHat cluster (north of the Pelican cluster) overlaps with relatively high-extinction regions (see Figure 14). It is located at the edge of the W80 bubble, and has ample molecular gas with which to continue forming stars and feeding the accretion of existing protostars. Indeed, a recent survey of outflows (Zhang et al. 2020) has revealed a few molecular outflows in PelHat, indicating active protostellar accretion. Meanwhile, we know that the Pelican cluster has little molecular gas. It is therefore probable that the pre-Pelican cloud was stripped from the Pelican cluster. Either the pre-Pelican cloud was pushed away by radiation pressure or bubble expansion, or the cloud was ionized by the Bajamar Star. In any case, the Pelican cluster should be closer to the Bajamar Star than PelHat. In other words, the cluster should be inside the bubble by now.

To carry out our basic feedback analysis, we need to estimate and make various assumptions about the H ii region and the pre-Pelican cloud. The ionizing photon flux from the Bajamar Star is S_UV ∼ 10⁴⁹ s⁻¹, and the electron density, n_e , of the H ii region is ∼10 cm⁻³ (Wendker 1968; Comerón & Pasquali 2005). The ionizing flux, S, from the Bajamar Star corresponds to a luminosity of 2.2 × 10³⁹ erg s⁻¹ (assuming 13.6 eV photon energy). The W80 bubble radius is ∼20 pc, much larger than the Strömgren radius of ∼2.8 pc (assuming a mean gas number density, n, of 100 cm⁻³ before ionization, see Equation (7.24) in Krumholz 2017). Hence, the bubble is likely to have expanded significantly. Assuming a temperature of 10⁴ K for the H ii region, the corresponding sound speed is ∼11 km s⁻¹. If we assume a mean stellar mass of 0.5 M_⊙ (Evans et al. 2009) for the Pelican cluster members, then the total cluster mass (with 247 members, R11) is ∼120 M_⊙. Assuming a star formation efficiency of 3% (Kennicutt & Evans 2012), the total pre-Pelican cloud mass M would be 4 × 10³ M_⊙,

Firstly, we consider that the pre-Pelican cloud was pushed by bubble expansion. Assuming that thermal pressure in the bubble dominates the expansion, then the total expansion time can be computed following Equation (7.32) in Krumholz (2017).

where r_i is the radius of the ionized sphere, S₄₉ is the ionizing photon flux (in units of 10⁴⁹ s⁻¹), n₂ is the initial number density before ionization (in units of 100 cm⁻³), T_i,4 is the temperature of the ionized gas (in units of 10⁴ K), and t₆ is the time of expansion (in units of 10⁶ yr). Adopting r_i = 20 pc, S₄₉ = 1, n₂ = 1, and T_i,4 = 1, the total expansion time, t, is about 3.7 Myr. Taking the time derivative of the equation, the current bubble expansion velocity is

which is about 3 km s⁻¹. The typical relative velocity between the molecular filaments in the NAN region and the W80 bubble is a few km s⁻¹. Bubble expansion is therefore a possible mechanism for the dispersal of the pre-Pelican cloud. Considering the geometry, it is possible that the molecular gas to the northwest of the Pelican Head is partly composed of gas from the pre-Pelican cloud that has been pushed to that position.

Secondly, we consider that the Lyα photons from the H ii region exert a radiation pressure on the molecular gas. Assuming that the solid angle subtended by the pre-Pelican cloud (as viewed from the Bajamar Star) is Ω, then the force caused by the radiation pressure is SΩ/4π c, where c is the speed of light. The final velocity of the pre-Pelican cloud after t is (SΩ/4π c)t/M. If we assume that Ω = 4π/10, the pre-Pelican cloud reaches a velocity of about 1.1 km s⁻¹, smaller than that found for bubble expansion.

Finally, we consider that the pre-Pelican cloud was ionized by the Bajamar Star, so that we do not see the molecular gas today. For an ideal Strömgren sphere, the ionized gas mass is S_UV μ m_H /n α_B , where μ = 1.4 is the mean particle weight, m_H is the hydrogen mass, n is the particle density, earlier assumed to be 100 cm⁻³ (see Equation (7.23) in Krumholz 2017), and α_B = 3.5 × 10⁻¹³ cm³ s⁻¹ is the case B recombination coefficient for hydrogen. Given the aforementioned flux, S_UV, the H ii region mass is about 3 × 10³ M_⊙, comparable to the pre-Pelican cloud mass M.

The actual history of the cloud dispersal may comprise a combination of bubble expansion, radiative pressure, and ionization. In other words, part of the pre-Pelican cloud may have been ionized, and the rest could have been pushed away. In any case, it is possible that the Bajamar Star and the W80 bubble together could have dispersed the pre-Pelican cloud.

The Gulf cluster, however, requires closer examination. There are probably three subclusters in this region. From west to east, Guieu et al. (2009, hereafter G09) named them clusters 1, 2, and 3 (see their Figure 9). Based on our combined molecular line data, we find evidence that the three subclusters are probably separated.

In Figure 17, we show the ¹³CO gas associated with the three G09 subclusters. In panel (c), the G09 cluster 3 is associated with a 24 μm dark region at the northeastern end of the Gulf region (the “Dark Crab”). It is embedded in highly extinct molecular gas at ∼4.9 km s⁻¹. Meanwhile, the G09 cluster 2 spatially matches the 24 μm dark region of the F-6 filament, at a velocity of ∼1.5 km s⁻¹. Consequently, the two G09 clusters are associated with two distinct molecular gas regions. As such, they are probably not part of the same cluster.

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More interestingly, the location of the G09 cluster 1 has no 24 μm dark counterpart, as shown in Figures 17 (b) and (c). This cluster nicely coincides with the location of the background F-7 filament (Figures 11 and B7). This is especially true at 7.1 km s⁻¹ (panel (d)). It is possible that the G09 cluster 1 is embedded in F-7, which lies on the far side of W80. Six members in cluster 1 have Gaia DR2 data: their distances are (in ascending order) 1103, 1111, 1611, 1863, 2457, and 3565 pc. These distances are significantly larger than the estimated distance to W80, and are therefore consistent with the scenario whereby this is a cluster background to W80.

Overall, we see that at least some of the star formation activity in the NAP complex is strongly impacted by feedback from the Bajamar Star.