First Embedded Cluster Formation in California Molecular Cloud

DRAFTVERSION MARCH 24, 2020 Typeset using LATEX twocolumn style in AASTeX62

First embedded cluster formation in California molecular cloud

JIN-LONG XU,1, 2 YE XU,3 PENG JIANG,1, 2 MING ZHU,1, 2 XIN GUAN,1, 2 NAIPING YU,1 GUO-YIN ZHANG,1 AND DENG-RONG LU3

1National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China 2CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China 3Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China

(Received xx xx, 2019; Revised xx xx, 2019; Accepted xx xx, 2019)

ABSTRACT We performed a multi-wavelength observation toward LkHα 101 embedded cluster and its adjacent 850× 600 region. The LkHα 101 embedded cluster is the first and only one significant cluster in California molecular cloud (CMC). These observations have revealed that the LkHα 101 embedded cluster is just located at the projected intersectional region of two filaments. One filament is the highest-density section of the CMC, the other is a new identified filament with a low-density gas emission. Toward the projected intersection, we find the bridging features connecting the two filaments in velocity, and identify a V-shape gas structure. These agree with the scenario that the two filaments are colliding with each other. Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we measured that the RRL velocity of the LkH 101 H II region is 0.5 km s−1, which is related to the velocity component of the CMC filament. Moreover, there are some YSOs distributed outside the intersectional region. We suggest that the cloud-cloud collision together with the fragmentation of the main filament may play an important role in the YSOs formation of the cluster.

Keywords: ISM: clouds – stars: formation – ISM: kinematics and dynamics – ISM: individual objects (LkHα 101)

1. INTRODUCTION invoked to explain the formation of super star clusters (Fu- rukawa et al. 2009; Fukui et al. 2014, 2016). High-mass stars (>8 M ) play an important role in pro- moting the evolution of their host galaxies (Zinnecker & California molecular cloud (CMC) presents a filamentary Yorke 2007). However, how the high-mass stars form is not structure extended over about 10 deg in the plane of sky yet well understood. Most high-mass stars usually form in (Lada et al. 2009). In the Perseus constellation, the CMC stellar clusters (Lada & Lada 2003). In order to understand is considered as the most massive giant molecular cloud in how a high-mass star forms, we must understand the forma- the range of 0.5 kpc from the Sun (Lada et al. 2009). Despite tion of its host embedded cluster. Recently, young embed- its large size and mass, the CMC appears to be very mod- ded cluster-forming systems are often found to be associ- est in its star formation activity. In the CMC, the LkHα 101 ated with the hub-like multiple filamentary structure. Hence, embedded cluster is the first and only one significant clus- the filament-hub accretion can be used to explain the for- ter, which is also consistent with the LkHα 101 H II region mation of the young embedded cluster (Myers 2009; Kirk (Barsony et al. 1990; Dzib et al. 2018). In the embedded et al. 2013). In addition, individual embedded clusters often cluster, the star LkHα 101 is a young high-mass star with a present small age spreads (Jeffries et al. 2011; Getman et al. spectral type of near B0.5 (Herbig et al. 2004; Wolk et al. 2014), indicating that a whole cluster of stars precipitate the 2010). Compared to the Orion nebula containing many OB arXiv:2003.09811v1 [astro-ph.GA] 22 Mar 2020 quasi-instantaneous formation (Elmegreen 2000; Tan et al. stars, Lada et al.(2017) suggested that the CMC is a sleep- 2006). This inquires that a significant amount of dense gas ing giant. Hence, the CMC provides us an ideal laboratory need to be rapidly gathered in the place where the cluster for investigating the formation of embedded cluster. In this was born. Collisions between molecular clouds and exter- Letter, we performed a multi-wavelength study to investigate nal effects (e.g., H II region and supernovae) can satisfy the the formation imprint of the LkHα 101 embedded cluster in condition for rapid gas accumulation (Elmegreen 1998; In- the CMC. oue & Fukui 2013). The cloud-cloud collisions have been 2. OBSERVATION AND DATA PROCESSING To show the molecular gas distribution surrounding the 0 0 [email protected], [email protected] LkHα 101 embedded cluster, we mapped a 85 × 60 region centered at position of the LkHα star in the transitions of 12CO J=1-0, 13CO J=1-0 and C18O J=1-0 lines using the 2

Purple Mountain Observation (PMO) 13.7 m radio telescope, during May and December 2019. The 3×3 beam array re- ceiver system in single-sideband (SSB) mode was used as -8.75° 10 front end. The back end is a fast Fourier transform spectrom- 20 30 eter (FFTS) of 16384 channels with a bandwidth of 1 GHz, 40 -9.00° 50 −1 20 30 corresponding to a velocity resolution of 0.16 km s for 0 10 12CO J=1-0, and 0.17 km s−1 for 13CO J=1-0 and C18O

12 Latitude Galactic J=1-0. CO J=1-0 was observed at the upper sideband -9.25° with a system noise temperature (Tsys) of ∼280 K, while 13 18 CO J=1-0 and C O J=1-0 were observed simultaneously (a) H2 column density -9.50° at the lower sideband with a system noise temperature of 165.75° 165.50° 165.25° 165.00° 164.75° 00 ∼150 K. The half-power beam width (HPBW) was 53 at Galactic Longitude 115 GHz. The pointing accuracy of the telescope was bet- ter than 500. Mapping observations use the on-the-fly mode with a constant integration time of 14 seconds at each point and with a 0.50 × 0.50 grid. The reference-point position is at -08.75° l= 165.2993◦ and b= -9.2197◦. The standard chopper-wheel method was used to calibrate the antenna temperature (Ulich -09.00° & Haas 1976). The calibration errors are estimated to be within 10%. Galactic Latitude Galactic Moreover, we observed the LkHα 101 H II region asso- -09.25° ciated with the LkHα 101 embedded cluster in the C168α (b) 12 μm (W3) (1375.28630 MHz) and H168α (1374.60043 MHz) radio -09.50° recombination lines (RRLs) using the Five-hundred-meter 165.75° 165.50° 165.25° 165.00° 164.75° Galactic Longitude Aperture Spherical radio Telescope (FAST), during Septem- Figure 1. 1.4 GHz radio continuum contours in black colour, over- ber 2019. FAST is located in Guizhou, China. The aperture laid on the Herschel H column density and WISE 12 µm images of the telescope is 500 m and the effective aperture is about 2 0 of the observed region in (a) and (b) panels, respectively. The black 300 m. The half-power beam width (HPBW) was 2.9 and −1 the velocity resolution is 0.1 km s−1 for the digital backend contours begin at 5σ in steps of 10σ, with 1σ = 0.7 mJy beam . at 1.4 GHz. Single pointing observation of the LkHα 101 H II The black arrows mark the positions of several pillars. The two region was taken with the narrow mode of spectral backend. green arrows mark the direction and position of position-velocity This mode has 65536 channels in 31.25 MHz bandwidth. diagram in Figure3. We also label the offsets (y-axis in Figure3) During observations, system temperature was around 18 K. over the two arrows. The observed position was integrated for 60 minutes. Re- duced root mean square (rms) of the final spectrum is about consistent with a filamentary dark cloud L1482 (Harvey et al. 11.0 mK. Jiang et al.(2019) gave more details of the FAST 2013; Li et al. 2014), which is the highest-density part of the instrumentation. CMC (Lada et al. 2017; Zhang et al. 2019). As marked by To trace the ionized-gas distribution of the LkHα 101 H II the two green long arrows in Figure1(a), we identify a new region, we also use the 1.4 GHz radio continuum emission filament with a low-density gas emission, named minor fil- data, obtained from the NRAO VLA Sky Survey (NVSS) ament. The minor filament appear to be perpendicular with with a noise of about 0.45 mJy/beam and a resolution of the main filament in the line of sight. 4500 (Condon et al. 1998). In order to trace the polycyclic The 1.4 GHz radio continuum emission can be used to aromatic hydrocarbon (PAH) emission, we also utilized the trace ionized gas. In Figure1(a), the ionized-gas emission 12 µm (W3) infrared data from the survey of the Wide-field in black contours of the LkHα 101 H II region shows a com- Infrared Survey Explorer (WISE; Wright et al. 2010). The pact structure, which is located at the intersectional region of infrared band 12 µm has the angular resolutions of 600. 5. the main and minor filaments. The WISE 12 µm contains PAH emission (Tielens 2008). The PAH molecules are ex- 3. RESULTS cited by the UV radiation from the H II region. In Figure1(b), 3.1. Infrared and Radio Continuum Images the PAH emission shows a bubble-like structure and several pillars, indicating that the expanding bubble created by the Figure1(a) shows an H 2 column density map for our ob- LkHα 101 H II region is interacting with the main filament. served region. Zhang et al.(2019) constructed the H 2 column density map with a resolution of 18.200 by using the Herschel Figure2 shows the C168 α and H168α RRLs spectra of the data toward the entire CMC. Here, we only cut out a 850× LkHα 101 H II region from the FAST observations. We use 600 region centered at position of the LkHα 101 star from the the double-Gaussians to fit the data. By comparing with the H168α RRL, the C168α RRL with a narrow line width is H2 column density map. In Figure1(a), the H 2 column density map shows a filamentary structure elongated from south- more suitable for tracing the RRL velocity of ionized gas. east to northwest, named main filament. The main filament is 3

α α

− − l Figure 2. The C168α and H168α RRLs spectra of the LkHα 101 H II region from the FAST observations. The red smooth curves is a double-Gaussian ﬁt to the observed spectra. The velocity resolution is smoothed to 0.2 km s−1, then the noise RMS is 7.8 mk.

Hence, we obtain the RRL velocity of 0.5±0.1 km s−1 for the LkHα 101 H II region. Figure 3. Position-velocity diagram of the 12CO J=1-0 emission 3.2. Molecular Gas Images along the minor filament, as shown in the black dashed line in Figure 1a. The red dashed line marks the systemic velocity of -0.7 km s−1 H2 column density only gives the distribution of projected 2D gas. Here, we use 12CO J=1-0, 13CO J=1-0, and C18O for the main filament. The purple dashed line indicates the RRL J=1-0 lines to trace the molecular gas distribution of the two velocity of 0.5 km s−1 for the LkHα 101 H II region. The blue filaments. To investigate the gas structure of the minor fila- dashed line indicates the systemic velocity of -6.0 km s−1 for the ment, we made a position-velocity (PV) diagram (Figure3) minor filament. The contour levels are 10, 20,..., 90% of the peak of the 12CO J=1-0 emission along the black dashed line and value. The bridging positions are indicated by green dashed lines. through the position of the LkHα star in Figure1. In Fig- The purple star represents the LkHα 101 H II region or LkHα 101 ure3, we see that there are two main velocity components, embedded cluster. The green circle indicate a molecular shell. A which range from -10.1 km s−1 to -4.0 km s−1, and from “V-shape” gas structure is indicated by the yellow dashed lines. -4.0 km s−1 to 4.1 km s−1. In 12CO J=1-0 emission, we can clearly see bridging features in velocity between the two filament. We did not detect the C18O J=1-0 emission in the components, indicated by the green dashed lines in Figure 12 whole minor filament, suggesting that the minor filament is 3. Moreover, the CO J=1-0 emission show a molecular not a filament like the main filament with dense gas. shell and a “V-shape” gas structure, which are indicated by Figures4(c) and4(d) give the spectra in the typical posi- the green circle and yellow dashed lines in Figure3. Us- tions of the two filaments. From the C18O J=1-0 spectrum, ing the two velocity ranges, we make the integrated inten- 12 13 18 we obtain that the systemic velocity of the main filament is sity maps of CO J=1-0, CO J=1-0, and C O J=1-0, -0.7 km s−1. Since we do not detect the C18O J=1-0 emi- overlaid with the 1.4 GHz radio continuum contours (pink) sion in the minor filament, the systemic velocity of -6.0 km and WISE 12 µm contours (green), as shown in Figures4(a) s−1 for the minor filament is determined by the 13CO J=1- and4(b). In Figure4(a), the velocity component of -4.0 km −1 −1 0 emision. The systemic velocities of the two filaments are s to 4.1 km s displays a filamentary structure elongated marked in blue and red dashed lines in Figure3, suggesting from southeast to northwest, which should be belong to the 12 that although the two filaments intersect each other, the in- main filament. For the main filament, the CO J=1-0 and tersection is characterized by two different velocity compo- 13 J CO =1-0 emission also show a shell-like structure, just nents. The LkHα 101 embedded cluster with the H II region surrounding the LkHα 101 H II region. From the PV dia- is just located at the projected intersectional region of the two gram, we also see the molecular shell. Compared with the filaments in the line of sight. 12CO J=1-0 and 13CO J=1-0 emission, the C18O J=1-0 emission may trace the dense part of the main filament. The C18O J=1-0 emission is likely to be cut into two parts at the 3.3. Young Stellar Object Distribution position of the LkHα 101 H II region. In Figure4(b), the Based on two young stellar object (YSO) catalogs, Lada velocity component of -10.1 km s−1 to -4.0 km s−1 shows a et al.(2017) summarized a new YSO table, which contains northeast-southwest filament, whose direction is almost per- 43 Class I YSOs, 116 Class II YSOs, and 7 Class III YSOs in pendicular to the main filament. Compared with Figure1(a), the whole CMC. Class I YSOs are protostars with circumstel- the northeast-southwest filament is associated with the minor lar envelopes and a timescale of the order of ∼ 105 yr, while 4

Figure 4. (a) and (b) panels: 1.4 GHz radio continuum contours (pink), WISE 12 µm contours (green), 13CO J=1-0 integrated-intensity contours (blue), and C18O J=1-0 contours (red) superimposed on the integrated intensity 12CO J=1-0 maps (grey). The integrated velocity ranges are indicated in the lower right corner of each map. The right grey bar is units of K km s−1. The contours begin at 5σ in each emission. (c) and (d) panels: The typical spectra of the positions are indicated by the white squares in Figures4(a) and4(b). Class II YSOs are disk-dominated objects with a ∼ 106 yr et al. 2017; Fukui et al. 2018). In addition, the main filament (Andre´ & Montmerle 1994). The Class III YSOs are the pre- shows a “V-shape” gas structure in the PV diagram, which main sequence stars. From the table, we found 18 Class I is similar to that shown in Figure 14 of Fukui et al.(2018). YSOs, 72 Class II YSOs, and 4 Class III YSOs in our ob- Fukui et al.(2018) suggested that the V-shaped protrusion in- served region. Figure5(a) shows the spatial distribution of cluding the bridge in a PV diagram provide a characteristic these YSOs. To investigate the positional relation of YSOs feature of collision, especially for that one of the colliding with the main and minor filaments, we overlaid the selected clouds is smaller than the other. We also know that the size YSOs on the 13CO J=1-0 emission map (green) of the main of the minor filament is significantly smaller than that of the filament, and the 12CO J=1-0 emission map (blue) of the main filament. For the main filament, the 12CO J=1-0 and minor filament. Class I and Class II YSO sources in Figure 13CO J=1-0 emission present a shell-like structure, which 5(a) are found to be mainly distributed along the main fila- surrounds a PAH bubble created by the LkHα 101 H II re- ment. The main filament contains ∼ 42% of Class I YSOs gion. The shell-like structure detected indicates that the ex- and ∼ 62% of Class II YSOs in the CMC while occupying panding PAH bubble created by the LkHα 101 H II region only a small fraction of the total cloud area. The main fil- is interacting with the main filament. However, from Figure ament includes most young stars, probably because it is the 3, we see that the LkHα 101 H II region is still confined to densest part of the CMC, and has a high gas density. In Fig- a molecular shell, which is shown in a green circle. Further- ure5(a), we can also clearly see that compared with other more, the two bridging features in Figure3 are not connected region in the main filament, the projected intersectional re- to the LkHα 101 H II region, indicating that the bridging and gion of the two filaments gathers more Class II YSOs with V-shape features are not created by the feedback of the LkHα about 1 Myr. 101 H II region. Hence, we further conclude that the main filament is colliding with the minor filament. 4. DISCUSSION 4.1. Dynamics structure 4.2. The LkH 101 embedded cluster formation

Through the Herschel H2 column density and CO emission In the CMC or main filament, the LkHα 101 embedded maps, we found the two filaments intersect each other in the cluster with an H II region and several B0.5 stars is the first line of sight. As shown in the Figure3, the main and minor and only one significant cluster (Barsony et al. 1990; Dzib filaments have different systemic velocities. It suggests that et al. 2018). We see that the LkHα 101 embedded clus- the intersection from the two filaments is characterized by ter is just located at the projected intersectional position of two different velocity components. This scenario indicates the main filamnet with the minor filament. The morphology that the two filaments is likely to be colliding with each other. can be alternatively explained by the filament-hub accretion We also find several pieces of evidence to support the collid- from the surrounding filament material, which was believed ing idea. In the 12CO J=1-0 PV map, the bridging features to play an important role in the formation of the young em- connect the two filaments in velocity, which indicates that the bedded cluster (Myers 2009; Kirk et al. 2013), however, this two filaments are interacting (e.g., Haworth et al. 2015; Torii does not seem to be the case for the LkHα 101 embedded 5

(a) embedded cluster. This inquires that a significant amount of dense gas need to be rapidly gathered in the position of the main filament where the LkHα 101 embedded cluster was born. Recent magnetohydrodynamic (MHD) numerical sim- ulations on cloud-cloud collision show that after the collision the local density can rapidly increases from 300-1000 cm−3 to 104 cm−3 (Inoue & Fukui 2013), suggesting that the cloud-cloud collision can provide help for an embedded cluster formation. Hence, we conclude that the collision between the main and minor filaments may play a certain role in the LkHα 101 embedded cluster formation. The system velocity of the LkHα 101 embedded cluster (b) is just located between -4.0 and 4.1 km s−1 for the velocity H II region LkHα 101 component of the main filament, indicating that the LkHα MainFilament 101 embedded cluster formed in the main filament. The formation circumstances of the LkHα 101 embedded cluster in -6.0 km/s the CMC is very similar to that of super star cluster RCW 38 (Fukui et al. 2016). The triggered clusters form in the main component, not in the interaction region of two molec- Minor Filament ular clouds. Figure5b shows a summary for the formation -0.7 km/s YSOs scenario of the LkHα 101 embedded cluster. A super star Dense gas cluster formation is triggered where two clouds collide at a velocity separation of 10-30 km s−1 (Fukui et al. 2016). For the main and minor filaments, the velocity separation is 5.3 −1 Figure 5. (a) panel: 1.4 GHz radio continuum contours (red colour) km s . Therefore, here cloud-cloud collision triggered the overlaid on the 13CO J=1-0 emission map (green) of the main fila- formation of the LkHα 101 embedded cluster with B stars, ment, and 12CO J=1-0 emission map (blue) of the minor filament. not a super star cluster. In Figure5(a), there are some YSOs distributed outside the The pink circles indicate the positions of Class II YSOs, the yellow intersectional region in the main filament. The filament can and white circles for Class I YSOs and Class III YSOs, respectively. be fragmented to form prestellar cores and protostars (Andre´ (b) panel: sketched diagram of the main and minor filaments. et al. 2010). Therefore, the evolution and fragmentation of the main filament may also plays an important role in some cluster formation in the main filament. We can adopt the YSOs formation of the LkHα 101 embedded cluster. How- −1 RRL velocity (0.5±0.1 km s ) of the LkHα 101 H II region ever, these YSOs don’t seem to form a real cluster, suggest- as the system velocity of the LkHα 101 embedded cluster, ing that except for the intersectional region, the other regions −1 −1 which is not located between -10.1 km s and -4.0 km s seem to have no conditions to produce an embedded cluster for velocity component of the minor filament. Hence, the in the main filament. Hence, we suggest that the cloud-cloud LkHα 101 embedded cluster has no direct contact with the collision together with the fragmentation of the main filament minor filament, and it is impossible for the minor filament to help the LkHα 101 embedded cluster formation in the CMC. provide material for the embedded cluster formation. Li et al. Although the present observations can explain the formation (2014) proposed that the formation of the embedded cluster of the LkHα 101 embedded cluster, the more observations was caused by the merging of the two sub-filaments in the at higher spatial resolution are needed to resolve the detailed main filament. It is very similar to the hub-system accretion kinematics in this region. model. Furthermore, if there is an accretion flow in the minor filament, such accreting flows will display self-absorption in 5. CONCLUSIONS optically thick emission (Lee et al. 2013; Kirk et al. 2013), 12 13 In this Letter, we present a multi-wavelength observation which is not found in our CO J=1-0 and CO J=1-0 data towards LkHα 101 embedded cluster and its adjacent re- in the main and minor filament, as shown in the Figures4 (c) gion. These observations have revealed that the embedded and (d), respectively. cluster are just located at the projected intersectional region In addition, the cross-like structure combined by the main of the main and minor filaments. The main filament is the and minor filaments seem to be similar to L1188. Gong et al. highest-density part of the CMC, while the minor filament is (2017) suggested that cloud-cloud collision happened in the a new identified filament with a low-density gas emission. In L1188, and triggered star formation in it. From Section 4.1, the 12CO J=1-0 PV map, the intersection shows the bridg- we know that the cloud-cloud collision exactly happened in ing features connecting the two filaments in velocity, and we the main filament. The age of the LkHα 101 embedded clus- also identify a V-shape gas structure. These agree with the ter would be 1-2 Myr (Herbig et al. 2004), suggesting that scenario that the two filaments are colliding with each other. the LkHα 101 embedded cluster appears to be an individual From the FAST observation, we obtained the RRL velocity 6 of 0.5±0.1 km s−1 for the LkHα 101 H II region, which is We thank the referee for insightful comments that im- just located between -4.5 to 4.1 km s−1 for the velocity com- proved the clarity of this manuscript. This work was sup- ponent of the main filament. We suggest that the collision ported by the Youth Innovation Promotion Association of between the two filaments may play a certain role the LkHα CAS, the National Natural Science Foundation of China 101 embedded cluster formation in the CMC. In addition, (Grant Nos. 11873019 and 11933011), and also supported there are also some YSOs distributed outside the intersec- by the Open Project Program of the Key Laboratory of FAST, tional region, this indicates that the evolution and fragmenta- NAOC, Chinese Academy of Sciences. tion of the main filament may also play an important role in the YSOs formation of the cluster.

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