Star formation rates in the L 1482 filament of the California molecular cloud

Abstract

We measured the trigonometric parallax of the H₂O maser source associated with the L 1482 molecular filament hosting the most massive young star, LkHα 101, in the California molecular cloud. The measured parallax is 1.879 ± 0.096 mas, corresponding to the distance of 532 ± 28 pc. This parallax is consistent with that of the nearby star cluster LkHα 101, which was recently measured with Gaia DR2. We found that the L 1482 molecular filament and the LkHα 101 cluster are located at the same distance within 3 ± 30 pc. We observed the southern parts of L 1482 molecular clouds including the H₂O maser source, which is adjacent to LkHα 101, using the Nobeyama 45 m telescope in the J = 1–0 transitions of both ¹²CO and ¹³CO. The peak intensity of the ¹²CO line revealed the high excitation temperature region (60–70 K) due to heating by UV radiation from LkHα 101. We derived the column density of these molecular clouds assuming local thermodynamic equilibrium (LTE) from the ¹³CO emission. Using Dendrogam, we searched for small-scale, dense structures (cores) and identified 337 cores in the ¹³CO data. Gravitationally bound cores with a virial mass to LTE mass ratio ≤1.5 and young stars are concentrated in the high excitation temperature region. The column density in the warm region is five to six times larger than that of the surrounding colder molecular region. This suggests that the warm region has been compressed by a high-pressure wave and successive radiation-driven star formation is in progress in this warm region. In the cold molecular cloud to the north of the warm region, the cores are likely gravitationally unbound, which may be the reason why star formation is not active there.

1 Introduction

The formation process of massive stars in giant molecular clouds is important but poorly understood phenomenon. As described by Lada, Lombardi, and Alves (2009), the California Molecular Cloud (CMC) rivals the well-known massive star-forming region, the Orion Molecular Cloud (OMC) as the most massive giant molecular cloud with similar mass, size, filamentary structure, and morphology. However, despite its large mass, the CMC is characterized by a star formation rate (SFR) that is an order of magnitude lower than that of the OMC. The most massive star that is forming in the CMC is an early B star, LkHα 101, embedded in a cluster of lower-mass young stars (Andrews & Wolk 2008). The study of star formation in the CMC is very important due to its marked contrast to that of the OMC. The CMC is characterized by filamentary structures, with L 1482 being one of the prominent filaments in the region. LkHα 101 is a member of the embedded star cluster of young low-mass stars in the south-eastern part of L 1482 and is likely an early B star (M ∼ 15 M_⊙, Herbig et al. 2004). The most prominent feature is a dark lane of L 1482 which runs across the southeastern corner of the LkHα 101 cluster (Andrews & Wolk 2008). Since the LkHα 101 cluster is at the junction of two molecular filaments, L 1482 filament 1 and filament 2 on the plane of the sky, Li et al. (2014) suggested that the origin of the LkHα 101 cluster might be merging molecular filaments fed by converging inflows. However, Redman et al. (1986) argued that the dark lane is in the foreground and probably not associated with the LkHα 101 cluster. That argument is further supported by the interstellar H⁺ chemistry constraints in the vicinity of an intense radiation source (Brittain et al. 2004). Accurate distances of both the LkHα 101 cluster and L 1482 filaments are required to understand star formation in the region.

Recently, the parallax of VLA J043001.15+337724.6, which is a member of the LkHα 101 cluster, was measured to be 1.87 ± 0.10 mas (corresponding to the distance of 532 ± 28 pc) with Very Long Baseline Array (VLBA) radio continuum observations (Dzib et al. 2018). The astrometric result of Gaia DR2 (Gaia Collaboration 2018) also contains the measured parallax of the LkHα 101 cluster. H₂O masers were detected in L 1482 filament 2 (Sunada et al. 2007), thus we made its parallax measurement with the VLBI astrometric observations to compare the distances of the LkHα 101 star cluster and the L 1482 molecular filament directly. If LkHα 101 is adjacent to L 1482, LkHα 101 (B star) and the intense radiation pressure from it will have a significant effect on the surrounding gas and form the H ii region. The cloud surface adjacent to the H ii region is photo-dissociated by the ultraviolet (UV) and far-UV (FUV). Recently, Goicoechea et al. (2016) observed

the Orion Bright Bar photo-dissociated region (PDR) using the ALMA telescope; the warm cloud edge, exposed to energetic radiation from the Trapezium stars, revealed a fragmented ridge of high-density substructures and photo-ablative gas flows. They suggest that the cloud edge has been compressed by a high-pressure wave moving into the molecular cloud. The warm PDR adjacent to LkHα 101 is a similar physical region to the Orion Bar PDR. In this warm region, successive star formation may be triggered due to radiation-driven star formation.

In this paper, we study the star formation in the warm region facing to the LkHα 101 cluster. We observed molecular regions adjacent to LkHα 101 using the Nobeyama 45 m telescope in the J = 1–0 transitions of ¹²CO and ¹³CO. By using the ¹³CO data, we derived the column density of the molecular clouds assuming local thermodynamic equilibrium (LTE). We executed Dendrogam analysis to identify gravitationally unstable dense cores. We explored the evolution of star formation in the California Molecular Cloud, We explored the evolution of star formation in the California Molecular Cloud, which Lada et al. (2009) referred to as a sleeping giant molecular cloud.

2 Observations and data reduction

2.1 VERA telescope

The VLBI observations of the 22.235080 GHz H₂O maser in L 1482 were carried out on 2010/333, 2011/016, 2011/113, 2011/228, 2011/297, and 2011/353 (year/day of the year) with VERA. The observing time was approximately eight hours at each epoch. VERA is a Japanese VLBI array, which consists of four 20 m diameter antennas in Mizusawa, Iriki, Ogasawara, and Ishigaki-jima, with 2300 km maximum and 1200 km minimum baseline lengths (Omodaka 2009) VERA has a dual-beam system, which allows simultaneously observing of adjacent sources (target maser and position reference sources within |${2{^{\circ}_{.}}2}$| separation) for the purpose of correcting for the tropospheric fluctuations (Kawaguchi et al. 2000). The target source, L 1482, and the position reference source, J0429+3319, which is listed in the second realization of the International Celestial Reference Frame (ICRF2), were simultaneously observed using the dual-beam system of VERA. The separation angle between the target and the position reference source is |${1{^{\circ}_{.}}84}$|⁠. The data correlation was carried out with the FX-type hardware correlator at the Mitaka campus of the National Astronomical Observatory of Japan (NAOJ) (Chikada et al. 1991). The phase tracking centers of L 1482 and J0429+3319 are |$(\alpha ,\,\delta )_{\rm J2000.0} = ({04^{\rm h}30^{\rm m}27{^{\rm s}_{.}}4008},\ + {35^{\circ }09^{\prime }17{^{\prime \prime }_{.}}649}$|⁠) and (⁠|${04^{\rm h}29^{\rm m}52{^{\rm s}_{.}}721121},\ + {33^{\circ }19^{\prime }01{^{\prime \prime }_{.}}85849}$|⁠), respectively. The accumulation time of the correlation process was set to 1 s. The frequency and velocity spacing of L 1482 are 15.625 kHz and 0.21 km s⁻¹, respectively. The total bandwidth and the frequency spacing of J0429+3319 are 240 MHz and 0.25 MHz, respectively. Data reduction was done using VERA Data Analyzer (VEDA), which is a software package developed by the Mizusawa VLBI Observatory of NAOJ. The fringe search of J0429+3319 was done with an integration time of 128 s and a time interval of 32 s. The fringe and the self-calibration solutions of J0429+3319 were applied accordingly to L 1482. The peak intensity in the self-calibration map of J0429+3319 is approximately 0.07 Jy beam⁻¹. The instrumental delay and phase between the dual-beam is calibrated by the horn-on-dish method using the artificial noise source and the phase correction detector (Honma et al. 2008b). The tropospheric delay is calibrated using the tropospheric zenith delay measured by GPS (Honma et al. 2008a) and the tropospheric mapping function (Niell 1996). The ionospheric delay is calibrated using the Global Ionosphere Map (GIM) produced by the University of Bern.

2.2 CO observations with the Nobeyama 45 m radio telescope

Using the Nobeyama 45 m radio telescope, the ¹²CO (J = 1–0) observations towards the 13^′ × 12^′ region around L 1482 was carried out between 2013 March and May, while the ¹³CO (J = 1–0) observations covering the 40^′ × 40^′ region in the southern part of L 1482 were done between 2013 December and 2014 January. We used the new two-beam, two-polarization, side-band separating receiver, TZ, for the ¹²CO (J = 1–0) observations, and the 25-element focal plane receiver, BEARS, installed in the 45 m telescope for the ¹³CO (J = 1–0) observations. At 110 GHz, the telescope has a beam size of 16^″ (half power beam width, HPBW). The beam separation of BEARS is |${41{^{\prime \prime }_{.}}1}$| on the plane of the sky (Sunada et al. 2000). The typical noise temperature was ∼150 K for the TZ receiver and 300 K for BEARS. The on-the-fly (OTF) technique was adopted to map each observation box. The temperature scale was determined by the chopper-wheel method. The telescope pointing was checked every hour by observing SiO maser line from Orion KL. The parameters of the observations are summarized in table 1.

3 Results

3.1 Annual parallax

A maser spot at V_LSR = 7.38 km s⁻¹ was detected over all six VLBI observation epochs. The peak intensities from the first to the last epochs were 2.37, 1.34, 1.80, 2.73, 9.21, and 7.24 Jy beam⁻¹. Figure 1 (left-hand panel) shows the phase-referenced map of this spot. The detected position of this H₂O maser spot, |$(\alpha ,\,\delta )_{\rm J2000.0} = ({04^{\rm h}30^{\rm m}27{^{\rm s}_{.}}4008},\ +{35^{\circ }09^{\prime }17{^{\prime \prime }_{.}}649})$|⁠, is consistent within |${0{^{\prime \prime }_{.}}2}$| with that of the class I young stellar object (YSO) J04302741+3509178 (Broekhoven-Fiene et al. 2015) in L 1482 filament 2 (Li et al. 2014), the bolometric luminosity of which is 9.26 L_⊙ (Harvey et al. 2013). The H₂O maser is most likely associated with this YSO. The positional variation of this maser spot during the six epochs is shown in figure 2. Least-square fits were made to these positions, with the parallax, as well as the proper motion. The parallax and the proper motion were obtained to be π = 1.879 ± 0.096 mas (D = 532 ± 28 pc) and (μ_αcos δ, μ_δ) = (3.17 ± 0.16, −8.55 ± 0.16) mas yr⁻¹, respectively. Systematic errors in (RA, Dec) of (0.127, 0.138) mas were added in quadrature to the formal errors calculated by σ_formal = θ_beam/(2 × SNR) to achieve the reduced χ² = 1.

3.2 CO excitation temperature

Figure 5 shows the spatial distribution of the ¹³CO (J = 1–0) spectra which we obtained by dividing the 40^′ × 40^′ observed region into 10 × 10 grids and by averaging the data in each grid. It is noted that the presence of an arc-shaped region (southward) with high excitation temperatures centering around LkHα 101, and we show the ¹³CO intensity distribution in figure 6. A further interesting feature is the presence of a single self-absorption over the region facing the LkHα 101 cluster. Figure 7 shows the comparison of the ¹²CO and ¹³CO spectra. Clear dips in the ¹²CO spectra can be recognized in the velocity range from −1 km s⁻¹ to +1 km s⁻¹. The velocities of these dips coincide with the peak velocities of the ¹³CO spectra. This suggests that the ¹²CO emission emitted from the warm region around LkHα 101 is absorbed by the colder gas in the foreground, because the line is optically thick.

As shown in the next subsection, we estimated the column densities of ¹³CO, |$N_{\rm ^{13}CO}$|⁠, on the assumption of LTE. We used T_ex derived from the ¹²CO data. However, the area mapped in ¹³CO is much larger than that mapped in ¹²CO. Li et al. (2014) carried out ¹²CO(J = 3–2) observations toward L 1482, and covered an extent similar to our ¹³CO map. They found that the strong ¹²CO(J = 3–2) emission is detected around the cloud (see, their figure 2) even in regions not covered by our ¹²COJ = 1–0 observations. Since the energy necessary to excite the J = 3–2 transition is hν/k ≃ 32 K in temperature, the regions emitting the molecular line should have an excitation temperature higher than this value. For regions not covered in our ¹²CO observations, we therefore assumed a uniform T_ex of 35 K.

3.3 Dense cores in L 1482 filament: core identification and mass derivation

In order to study the star formation activity in the L 1482 filament, we identified cores using the fiducial core-finding algorithm Dendrogram, (Rosolowsky et al. 2008) implemented in the astrodendro python package. Dendrogram is a tree diagram that represents the hierarchy of structures within given data. It is composed of two types of structure, “branches” and “leaves.” Branches are the structures that split into multiple sub-structures, and the leaves are the structures that have no sub-structure. In this study, we define the smallest structure “leaf” as a core, and focus on the detection of cores. We used the three-dimensional ¹³CO data cube, which is an appropriate tracer to analyze internal structures through the entire molecular cloud, because the ¹³CO emission line is moderately optically thin in the observed region, and is a good probe of the column density. To identify the cores, we applied Dendrogram to the ¹³CO cube with parameters min_value of 5σ, min_delta of 3σ, and min_npix of 5.2 pixels (⁠|$=A_{\theta _{\rm beam}}/A_{\rm pixel}$|⁠). We note that min_npix is the minimum number of pixels that the structures must contain. We used the signal-to-noise ratio map to avoid detection of fake sources due to the non-uniform noise distribution. The application of Dendrogram with these parameters to the ¹³CO data cube resulted in the identification of 337 cores in a square region of ∼40^′ × 40^′.

In table 2, we list the identified 337 cores in order of right ascension (J2000.0) and present their physical properties derived in the above. Figure 8 shows their locations. The radius of the identified cores ranges from 0.029 to 0.180 pc with a mean value of ∼0.054 pc. The core mass ranges from 0.76 to 35.49 M_⊙. We show the summary of the physical parameters of the core in table 3.

Finally, we also derived the total molecular mass of the entire region mapped in ¹³CO (figure 6) to be ∼7600 M_⊙ using the derived T_ex and equations (2) and (3).

4 Discussion

4.1 The relation between L 1482 filament and LkHα 101

The measured parallax of the H₂O maser in L 1482 filament 2 is consistent with that of the LkHα 101 cluster measured by VLBA and Gaia DR2. The difference is only 0.01 ± 0.10 mas. The parallax and the proper motion of VLA J043001.15+337724.6 in the LkHα 101 cluster is measured to be π = 1.87 ± 0.10 mas and (μ_αcos δ, μ_δ) = (1.86 ± 0.04, −5.70 ± 0.05) mas yr⁻¹ with VLBA radio continuum observations (Dzib et al. 2018). The parallax and the proper motion of LkHα 101 cluster using Gaia DR2 data (Gaia Collaboration 2018) are π = 1.876 ± 0.051 mas and (μ_αcos δ, μ_δ) = (2.44 ± 0.55, −5.27 ± 0.54) mas yr⁻¹, respectively. These are the averages of the parallaxes and the proper motions of 35 sources which are selected from the following three steps:

• We selected 178 sources located within 5^′ from the center of the LkHα 101 cluster |$(\alpha ,\,\delta )_{\rm J2000.0} = ({04^{\rm h}30^{\rm m}11{^{\rm s}_{.}}04},\ +{35^{\circ }16^{\prime }44{^{\prime \prime }_{.}}0})$|⁠.

• We selected 38 sources whose parallax errors are less than |$10\%$|⁠.

• We excluded three foreground sources with parallaxes 3.8, 5.0, and 13.5 mas.

Finally, 35 sources were selected. Figures 9 and 10 show distributions of the parallax and proper motion of the 35 sources. There is a difference of approximately 3 mas yr⁻¹ (8 km s⁻¹ at 532 pc) in the proper motions between the H₂O maser spot and the LkHα 101 cluster. While the H₂O maser spot is detected at V_LSR = 7.38 km s⁻¹, the ambient molecular gas in L 1482 filament 2 is detected at V_LSR ≃ −1 km s⁻¹ in the ¹³CO (J = 1–0) line (Li et al. 2014). The difference in velocity is approximately 8 km s⁻¹, which may originate from the internal motion of the H₂O maser spot with respect to the associated YSO and/or the peculiar motion of the associated YSO itself. The standard deviations of proper motions of the 35 sources in the LkHα 101 cluster are calculated to be 3.27 mas yr⁻¹ and 3.20 mas yr⁻¹ in RA and Dec, respectively. This indicates a velocity dispersion of approximately 8.2 km s⁻¹. However, two outlier sources which have large peculiar motions of 36 km s⁻¹ and 50 km s⁻¹ are included in the 35 sources, and these would lead to the overestimation of the velocity dispersion. We excluded two outlier sources and calculated the standard deviations to be 1.40 mas yr⁻¹ and 1.22 mas yr⁻¹ in RA and Dec, respectively, indicating the velocity dispersion of approximately 3.3 km s⁻¹. This is close to the average velocity dispersion of 4.5 km s⁻¹ calculated for 28 OB-associations (Melnik & Dambis 2020).

4.2 The arc-shaped warm region

It is noted that the warm arc-shaped region surrounds LkHα 101 in the center. The L 1482 molecular filament and the LkHα 101 star cluster are located at the same distance within 3 ± 30 pc. This suggests that the warm arc-shaped region is located at the periphery of the H ii region ionized by LkHα 101. In order to study the relationship between the warm molecular region and the H ii region, we superposed the VLA 1.4 GHz map (taken from Condon et al. 1998) on the ¹³CO integrated intensity map (see figure 11), which clearly shows that the arc-shaped region is heated by the FUV (6 eV < hν < 13.6 eV) radiation from LkHα 101. The arc-shaped region appears similar to the bright bar in Orion, and can be an ideal region to study photon-dominated region and star formation induced by the interaction with the H ii regions created by massive stars.

The thickness of the warm region is about 3^′, corresponding to ∼0.46 pc at the distance of 532 pc. Early theoretical model of PDRs (Tielens & Hollenbach 1985a, 1985b; Köster et al. 1994) estimated that the thickness of the region heated by the FUV corresponds to A_V = 10 mag in visual extinction, being only 0.03 pc assuming hydrogen gas densities |$n_{\rm H_2}$| of 10⁵ cm⁻³. As the observed ¹²CO emission line in PDR regions requires very high hydrogen gas densities, |$n_{\rm H_2}=10^{6}$|–10⁷ cm⁻³ (Burton et al. 1990). The emission from the PDR regions facing LkHα 101 may be produced in dense cores embedded in lower-density gas. In the warm region, we often observe a clear dip in the ¹²CO spectra. As shown in figure 7, the radial velocity of the dip nicely matches with the peak velocity of the optically thinner ¹³CO emission, indicating that the dip is due to the absorption by a large amount of colder gas lying in the foreground of the warm region. Because the dip is widely seen around the arc-shaped region, the high-temperature gas should be distributed over a considerable extent.

4.3 Star formation activities in the L 1482 filament

As noted in subsection 3.3, we identified 337 cores using Dendrogram analysis. Figure 12 shows the relation between M_vir and M_LTE of the 337 cores. As seen in the figure, the virial ratio α_vir( = M_vir/M_LTE) is greater than 1. In general, in case there is no external pressure (P_ext = 0), cores with α_vir < 1 are gravitationally unstable and are to collapse, while cores with α_vir > 1 are gravitationally unbound and are to disperse. Shimoikura et al. (2019) who studied massive cores in M 17 suggested that the cores with α_vir > 1 can also be gravitationally bound, if they are surrounded by high external pressure.

Since the region observed in this study is adjacent to LkHα 101 and a part of the region should be located in the high external pressure of the H ii region, some of the cores we detected may be gravitationally bound. Virial ratios of spherical cores with uniform density can reach α_vir = 2 at most, if they are in the gravitational equilibrium in high external pressure. However, the cores we detected are not perfectly spherical or uniform, and thus we tentatively assume in this paper that cores with α_vir ≤ 1.5 are gravitationally bound and that cores with α_vir > 1.5 are gravitationally unbound. As seen in figure 8, there are seven cores with α_vir ≤ 1.5 which are indicated by the blue circles in the figure, and the other cores with α_vir > 1.5 are indicated by the red circles. Three of the seven cores are located in the photo-dissociated region facing LkHα 101, and the other four cores are found in the northwest of LkHα 101. As is clear from figure 8, the ¹³CO emission is relatively weak in the northwest region between LkHα 101 and the four cores in the northwest, and the molecular density and therefore the external pressure is likely to be low there. It is noted that the size of the other three cores with α_vir ≤ 1.5 adjacent to LkHα 101 is very small compared with the four cores in the northwest, suggesting that the cores close to LkHα 101 are compressed by the high pressure in the H ii region. We imagine that these cores may eventually form stars when they become unstable due to increase of the external pressure, e.g., the impact of ionization front of the H ii region or strong stellar wind from LkHα 101.

Filaments are noted as the earliest stages of star formation. Theories and models for instabilities and fragmentation of the filament have been extensively studied (e.g., Ostriker 1964; Inutsuka & Miyama 1992). In contrast to the results of those studies, our results show that star formation is unlikely to occur in the massive filamentary structure L 1482 except for the southeast part facing LkHα 101. In the southeast part of the filament adjacent to LkHα 101 and the associated young cluster, the photo-dissociated warm region is formed, and the three cores with α_vir ≤ 1.5 are located therein. In this region, there are also protostars associated with H₂O masers. These results suggest that radiation-driven star formation triggered by the expansion of the H ii region is proceeding in the southeastern part of the CMC.

Bertoldi and McKee (1992) proposed an α_vir ∼ M^−0.67 relation. To test the validity of the exponent of the virial parameter, we made the plot of the core masses against their corresponding α_vir (see, figure 13). Our regression fit produced an exponent of −0.57. This is much closer to the −0.54 obtained for the Cepheus cloud (Bertoldi & McKee 1992). This is an indication that L 1482 could be a pressure-confined clump with the possibility of becoming a site of active star formation in the future.

Recently, an ALMA view of the Orion bright bar directly exposed to the Trapezium stars reveals a fragmented ridge of high-density substructures, photo-ablative gas flows, and instabilities at the molecular cloud surface (Goicoechea et al. 2016). Our results show the existence of the high-temperature gas in the warm region where clumpy structures are detected. The possible excitation scenarios of the warm gas are as follows;

• the ejection of energetic electrons from dust grains by FUV photons,

• turbulent heating, where the energy stored in local velocity dispersion is thermalized in small-scale shocks (Guesten & Fiebig 1988), and

• photo-ablative high-temperature gas flows as observed in the Orion bar.

At the moment, it remains uncertain which mechanism is the most effective. Also, Goicoechea et al. (2016) suggested that the cloud edge has been compressed by a high-pressure wave that currently moves into the cloud. Our results are consistent with this interpretation. The reasons are as follows;

1. the warm regions are formed adjacent to B star,

2. the column density in the warm region is 5–6 times higher than that of surrounding colder regions, and

3. protostellar cores, an H₂O maser source, and young stars are concentrated in this region.

This suggests that radiation-driven star formation is proceeding in this region.

Finally, we discuss the differences in star formation rates in the CMC and OMC. As noted by Lada et al. (2009), the CMC is very similar in morphology to the Orion A molecular cloud. However, the CMC shows an SFR that is an order of magnitude lower than that of the OMC. Figure 14 shows the 350 μm maps of the CMC (right-hand panel) and OMC (left-hand panel). The map of the CMC is enlarged to the scale of the distance of the OMC. As can be seen in figure 14, there are a number of dense and prominent filaments in the OMC which are known sites of massive star formation, whereas filaments in the CMC are apparently much less dense. According to Krumholz and McKee (2008), a high column density greater than ∼1 g cm⁻² [or N(H₂) ≃ 10²³ cm⁻²] is needed for the formation of massive stars. Formation of massive stars should be closely related to the high SFR through the compression of the natal clouds by the powerful stellar wind and the creation of the PDR regions. The critical density (∼1 g cm⁻²) corresponds to N(¹³CO) ≃ 2 × 10¹⁷ cm⁻². Nakamura et al. (2019) recently analyzed the ¹³CO data observed toward the OMC and found that roughly ∼10% of the total mass is confined in dense regions with N(¹³CO) > 2 × 10¹⁷ cm⁻² (see, their figure 27). The fraction of the high-density regions is much larger in the northern part of the OMC than in the diffuser, and more extended southern part which forms much less massive stars. A similar trend is also found in clouds associated with the M 17 H ii region and in rather quiescent infrared dark clouds in the vicinity known as M 17-SWex (Nakamura et al. 2019; Nguyen-Luong et al. 2020). In the case of the CMC, we analyzed the frequency distribution of N(¹³CO) using our data, and found that there are a very limited number of pixels (only 39 pixels) exceeding the critical value N(¹³CO) = 2 × 10¹⁷ cm⁻², and also found that such pixels are located only around the LkHα 101 cluster. This indicates that the mean density of the CMC is not high enough to produce massive stars, which can be one of the major reasons for the low SFR of the CMC compared with the OMC.

5 Conclusions

We have measured the trigonometric parallax of the H₂O maser source associated with the L 1482 molecular filament to be 1.879 ± 0.096 mas, corresponding to the distance of 532 ± 28 pc. This parallax is consistent with that of the nearby star cluster, LkHα 101. Our ¹²CO observations revealed the clumpy arc-shaped, warm structure illuminated by the LkHα 101 cluster. Based on the ¹³CO observations, we performed core identification, and found 337 cores. Most of the cores are likely to be gravitationally bound and thus they are unlikely to form stars except for some cores located along the arc-shaped structure. The column density in the warm arc-shaped region is 5–6 times higher than that of surrounding colder molecular region, which supports the idea that the radiation-driven star formation triggered by the expansion of the H ii region is occurring around the LkHα 101 cluster.

We have also compared the CMC and the OMC, and found the presence of more filamentary structures in the OMC. The OMC has a significantly higher star formation rate compared to the CMC, and the difference may be caused by the fact that nearly all the cores in the L 1482 filament of the CMC are gravitationally stable or unbound and thus they are unable to form stars. We also discussed that the mean density of the CMC is not high enough to produce massive stars, which can be one of the main reasons for the low SFR of the CMC compared with the OMC.

Acknowledgements

We thank to the anonymous referee for the useful comments to our manuscript. Data analysis was [in part] carried out on the Multi-wavelength Data Analysis System operated by the Astronomy Data Center (ADC), National Astronomical Observatory of Japan

References