NANTEN Observations of Dense Cores in the Corona Australis Molecular Cloud

PASJ: Publ. Astron. Soc. Japan 51, 911-918 (1999)

Yoshinori YONEKURA Department of Earth and Life Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531 E-mail (YY): [email protected] and Norikazu MIZUNO, Hiro SAITO, Akira MIZUNO, Hideo OGAWA* and Yasuo FUKUI Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya, ^64-8602 Downloaded from https://academic.oup.com/pasj/article/51/6/911/1467816 by guest on 30 September 2021

(Received 1999 August 26; accepted 1999 October 24) Abstract We carried out a C180 survey for dense molecular cores in the Corona Australis (CrA) molecular cloud with the NANTEN telescope. We observed 2.2 deg2 at a 2' grid spacing with a 2/7 beam, and 1980 positions were observed. We identified 8 C180 cores, whose typical line width, average column density, radius, mass, and average number density were 0.66 km s_1, 1.1 x 1022 cm-2, 0.13 pc, 18M®, and 1.4 x 104 cm-3, respectively. We found that AKomp, (Af(H2)), R, M, and n(H2) become larger along with an increase in the star-formation activity, whereas the ratio Mv-ir/M becomes smaller. A comparison of the present cores with those in Chamaeleon, Lupus, Ophiuchus, Taurus, and L 1333 indicates that star-forming cores tend to have a high column density, as well as a smaller MV-1T/M ratio. Key words: ISM: clouds — ISM: individual (Corona Australis Cloud) — ISM: molecules — stars: formation — radio lines: ISM

1. Introduction T CrA (FOe), TY CrA (B9e), and W CrA (Kl); there are also > 20 embedded IR sources, including the 'coro Recent observational studies have revealed that there net' cluster (Taylor, Storey 1984; Graham 1992; Wilking are a great variety of star-formation activities; only low- et al. 1992; Wilking et al. 1997). The brightest star in mass stars are born in dark clouds, whereas stellar groups the cloud is a Herbig Be star, TY CrA. These signa and massive stars are born in giant molecular clouds. tures of young stars make the CrA cloud one of the best One would expect that the star-forming activity in a regions to study intermediate-to-low-mass cluster forma cloud is correlated with the properties of the dense cores, tion along with the p Oph cloud. Although a number of since dense cores are the sites of present star formation. molecular line studies have been made toward the cloud Such a correlation, however, is still not observationally (Loren 1979; Loren et al. 1983; Levreault 1988; Harju et established, partly because of the lack of a rich sample al. 1993; Anderson et al. 1997), they are limited to the of dense cores. It is thus important to investigate the re densest part of the cloud, i.e., the R CrA cloud. lationship between the physical properties of dense cores We have carried out a survey for dense cores toward and the star-formation activities based on a rich sample the entire CrA cloud in ClsO (J = 1-0) emission at of dense cores in a systematic, statistical manner. 2.7 mm wavelength. In this paper, we present the physi The Corona Australis (CrA) molecular cloud is one cal properties of dense cores in the CrA molecular cloud of the nearest star-forming molecular clouds at 130 pc and compare the star-formation activity with the physi (Marraco, Rydgren 1981). The overall structure of the cal properties of dense cores. We also compare the char cloud has been investigated by studies of visual extinction acteristics of dense cores with those in the other nearby (Rossano 1978; Andreazza, Vilas-Boas 1996; Cambresy star-forming regions, such as Chamaeleon, Lupus, Ophi 1999). The cloud is highly elongated, having a dense uchus, Taurus, and L 1333. 'head' in the west (usually called the R CrA cloud) and a diffuse 'tail' in the east. Star formation is mainly taking 2. Observations place in the R CrA cloud. There are a number of pre- main-sequence stars, such as R CrA (A5e), S CrA (K6), C180 (J = 1-0) observations were made with the * Present address: Department of Earth and Life Sciences, Osaka NANTEN millimeter-wave telescope of Nagoya Univer Prefecture University, Sakai, Osaka 599-8531. sity at Las Campanas Observatory of Carnegie Institu-

© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System 912 Y. Yonekura et al. [Vol. 51, tion of Washington from 1998 February to March. The main-dish diameter of the NANTEN telescope is 4-m, providing a half-power beam width of 2/7 at 110 GHz, corresponding to ~ 1 pc for the distance of the CrA molecular cloud, 130 pc. The front-end was a 4 K cooled SIS mixer receiver (Ogawa et al. 1990). The typical sys tem temperature was 140 K (SSB) at 110 GHz, includ ing the atmosphere toward the zenith. The spectrometer was of the acousto-optical type with a total bandwidth of

3. Results and Discussion Fig. 1. Integrated intensity map of ClsO (J = 1-0) 18 shown in galactic coordinates. Equatorial coordi 3.1. Identification of C O Cores nates are also presented. The contour levels are 0.24 Figure 1 shows the total intensity distribution of the (3 a), 0.48, 0.96, 1.92, and 3.84 K km s"1. The dots ClsO (J = 1-0) spectra of the CrA molecular cloud. indicate the observed positions. We covered ~ 2.2 deg2, corresponding to ~ 11 pc2 at a distance of 130 pc. The map shows the existence of sev eral molecular condensations other than the main con of the ClsO cores are listed in table 1. densation, the R CrA cloud (Core 2, see table 1). In figure 2, we show a series of channel maps integrated 3.2. Physical Properties of Dense Cores over a velocity interval of 0.5 km s_1 in a velocity range -1 The physical properties of dense cores were estimated of 3.5 < FLSR < 7.5 km s . The velocity structure is in the following manner. For simplicity, we assumed the significant in Core 2; the westernmost part of Core 2 is _1 local thermodynamical equilibrium (LTE). We assumed ~ 1 km s redshifted from the rest of the cloud. This that the molecules along the line of sight possess a uni result is consistent with previous studies (Loren 1979; form excitation temperature, that the lines of the iso- Harju et al. 1993). topomers are at the same excitation temperature, and In order to study the physical properties of the dense 18 that the beam-filling factors are unity, although the less- regions, we define a C 0 core in the same manner as that abundant isotopes might be subthermally excited or their adopted by Onishi et al. (1996): (1) find a peak-intensity emission might arise primarily from the cloud interiors position, (2) draw a contour at a half level of the peak where the excitation conditions are different. In order intensity, (3) identify a core unless previously identified to estimate the excitation temperature, Tex, we observed cores exist within the half-level contour, (4) find the next the entire CrA cloud in 12CO (J = 1-0). Details of the intensity peak outside the core, (5) repeat the procedure 12CO observations will be published in a forthcoming pa after (2) until the peak intensity falls down below 6 a 1 per. We estimated Tex using the equation of radiative level (= 0.48 K km s" ). transfer for the optically thick line We finally identified 8 C180 cores. In figure 3, we show only the positions of the cores; the observed properties 5.53 (K), (1) ln{l + 5.53/[T£(12CO)(K) + 0.819]}

Table 1. Observed properties of C180 cores in CrA.

Position

Core number 1 6 a(1950) (5(1950) TK VLSR AV 1 1 (°) (°) /h m s\ (° ' ") (K) (kms" ) (km s" )

1 359.53 -20.47 19 10 15.0 -38 17 12 0.6 5.5 0.6 2 359.97 -17.80 18 58 20.3 -36 58 35 3.9 5.6 1.3 3 0.03 -18.93 19 03 45.9 -37 19 18 1.6 5.2 0.5 4 0.23 -18.77 19 03 18.9 -37 05 09 0.9 5.3 0.7 Downloaded from https://academic.oup.com/pasj/article/51/6/911/1467816 by guest on 30 September 2021 5 0.37 -19.50 19 06 59.1 -37 13 23 3.3 5.5 0.7 6 0.90 -20.43 19 12 14.7 -37 04 05 0.8 6.0 0.6 7 0.93 -20.17 19 11 02.3 -36 56 57 2.2 5.7 0.4 8 1.33 -20.50 19 13 14.2 -36 42 28 2.1 5.5 0.5

I 1 ' ' ' I ' ' ' ' I 1 1 1 L 3.5-4.0 kms" 4.0-4.5 kms"' 4.5-5.0 kms" 5.0—5.5 kms"

-1.0

0.5

-0.0 Kkms"1 1.0 pc

HPBW

_L_ -17 1 ' ' I ' I 1 I • • 1 ' ' ' I ' ' ' ' I . 5.5—6.0 km s 1 1 6.0-6.5 kms"1 6.5-7.0 km s"1 7.0-7.5 km s"1

CoO a

I ... i I • i .... i 1 0 Galactic longitude (Degree)

Fig. 2. Velocity channel maps of the CrA molecular cloud in ClsO (J = 1-0). Each map is integrated over a velocity interval of 0.5 km s_1. The maps cover the LSR velocities VLSR from 3.5 to 7.5 km s_1. The contour levels are 0.2, 0.4, 0.8, and 1.6 K kms"1.

01 km s T 2.42 x i0i4y-( ( ~Vi8(^) ex(K) exp[-5.27/T (K)] v *• ~ ex (cm-2). (3)

14 We also assumed 7V(H2) = [AT18/(1.7 x 10 ) + 1.3] x 1021 (Frerking et al. 1982). The mass of each core was estimated using

o 2 o M = 2.8mH ^[D fiAT(H2)], (4) i_ Downloaded from https://academic.oup.com/pasj/article/51/6/911/1467816 by guest on 30 September 2021 CD where D is the distance of the cloud, Q is the solid an CD "D gle subtended by the effective beam size, i.e., the grid spacing (2' x 2'), ran is the proton mass, and the fac tor of 2.8 is the mean molecular weight per H2 molecule. The summation was performed over the observed points O within the core. The radius of the core, R, was calculated CO from

R = (5)

where S is the area inside the core. We also calculated the virial mass, Mvir,

AVr, 1 o (6) kms -1 Galactic longitude (Degree) with AVcomp defined as the FWHM line width of the com Fig. 3. Positions of the cores indicated on a C180 in posite profile derived by using a single Gaussian fitting, tegrated intensity map. The contour levels are the where the composite profile was obtained by averaging same as in figure 1. all of the spectra within the core. The average H2 den sity, n(H2), in the core was derived by dividing M by the volume of the core based on the assumption that the core 12 is spherical with radius R. since the CO emission is usually optically thick and 18 thermalized even at low densities (~ 103 cm-3). For The physical properties of the C 0 cores, as well as most of the observed regions, the estimated excitation the number of associated young stellar objects, are listed temperature ranged from 8 to 11 K, and thus we adopted in table 2. The distribution of young stellar objects is shown in figure 4. The total mass of the molecular gas the average value of 10 K as Tex. For ~ 40' x 80' region 18 around Core 2, i.e., b > —18.7°, the estimated excita traced by C 0 in the observed area is estimated to be tion temperature ranged from 12 to 18 K, and thus we - 900 M®. adopted the average value of 15 K as Tex in the region. In order to derive the C180 column densities, we di 3.3. Properties of Dense Cores vided the spectrum into 0.1 km s_1 bins, calculated the In this subsection, we discuss the differences between column density within each bin, and summed them up the physical properties, such as the mass and radius, of within a velocity range in which the intensity is larger star-forming cores and those without signs for star for than 3 a. The peak optical depth of the C180 line in mation. We divided the cores into 3 groups: (a) the each bin, Ti$(V), was calculated using active star-forming core (Core 2), (b) the less-active star- forming core (Core 5), and (c) those cores without star T1S(V) formation (others). Table 3 gives the average physical TI8(10 =-In 1- (2) 5.27 {J18[rex(K)]-0.166} properties of these 3 groups as well as the average of all where Tx$(V) is the average temperatures of the C180 cores in the CrA molecular cloud. Figure 5 shows a series spectrum in each bin in the units of kelvins, and of histograms of the core parameters, such as [a] the line Ji [T(K)] = l/{exp [5.27/T(K)] - 1}. The ClsO col- width AVcomp, [b] average H2 column density (iV(H2)), 8 [c] radius R, [d] mass M, [e] average H2 number den sity n(H2), and [f] the ratio Mv-lr/M. It is clear in the

Table 2. Physical properties of C180 cores in CrA.

t Core no. 1 ex 7-1*8 iV18 (7V(H2))t n(H2) Mass R ^ Vcomp Mvir Mv„/M Number of 1 (K) (M«) (PC) (kms" ) (Afc) YSOs* 1.. 10 0.10 0.3 3.2 10.4 1.2 0.07 0.54 4.5 3.7 0 2.. 15 0.41 6.4 26.6 27.7 102.2 0.23 1.36 90.5 0.9 29 3.. 10 0.26 0.7 4.0 6.7 6.2 0.15 0.54 9.0 1.5 0 4.. 10 0.14 0.6 3.7 5.8 6.1 0.15 0.64 13.2 2.2 0 5.. 10 0.68 2.8 11.9 24.0 12.2 0.12 0.67 11.3 0.9 2 6.. 10 0.13 0.4 3.6 9.1 2.3 0.10 0.63 7.9 3.5 0 Downloaded from https://academic.oup.com/pasj/article/51/6/911/1467816 by guest on 30 September 2021 7.. 10 0.40 0.8 5.0 9.5 5.8 0.13 0.41 4.5 0.8 0 8.. 10 0.36 1.0 6.8 17.3 4.4 0.10 0.49 4.8 1.1 0

*The optical depth and column density are for the peak position of each core. f 15 2 21 -2 3 3 Units for iVi8, (iV(H2)>, and rc(H2) are 10 cm" , 10 cm , and 10 cm" , respectively. *Chen et al. (1997)

smaller along with an increase in the star-formation ac tivity, i.e., star-forming cores tend to be more deeply gravitationally bound. Here, we compare the properties of ClsO cores with those in Chamaeleon (Mizuno et al. 1999), Lupus (Hara et al. 1999), Ophiuchus (Tachihara et al. 1999), Taurus (Onishi et al. 1996, 1998), and L 1333 molecular cloud (Obayashi et al. 1998) observed with the same telescope. The cores in p Oph, Taurus, and Chamaeleon I can be re garded as samples of active star-forming regions, whereas those in Ophiuchus-north, Lupus, Chamaeleon III, and L 1333 are samples of less-active regions. Table 4 gives the average properties of dense cores in nearby star- forming regions. We find that the H2 column densities of the ClsO cores in the active star-forming regions are larger than those in less-active regions, whereas no clear

difference is recognized in Aycomp, R, and M. This result is consistent with previous studies, and thus it is further supported that the higher column density is a general trend for star-forming cores.

We also investigated the ratio of the virial mass, Mvir, to the LTE mass, M. For all star-forming cores in

CrA, Mvir/M is smaller than 1. This is consistent with previous studies. In L 1333, the cloud associated 1 0 with the youngest IRAS source, has the smallest ra Galactic longitude (Degree) tio, M^r/M, of ~ 1 (Obayashi et al. 1998). They sug gested that star formation may occur preferentially in a Fig. 4. Distribution of young stellar objects indicated cloud whose internal kinetic energy is the smallest com on a ClsO integrated intensity map. The contour level is 0.24 Kkms"1. pared with the self-gravitational energy. In Ophiuchus, all of the star-forming C180 cores are distributed over

a range of MvlT/M < 1 (Tachihara et al. 1999). In Chamaeleon, star-forming cores are almost in virial equi figure that AV^omp, (Af(H2)), R, M, and n(H2) become librium (Mizuno et al. 1999). In Lupus, star-forming larger along with an increase in the star-formation ac cores tend to have a smaller ratio in MwlT/M (Hara et tivity. On the other hand, the ratio Mvir/M, which will al. 1999). The present results support the idea that star be the indicator of the gravitational stability of the cores formation tends to occur in a cloud with a small MvxxjM (Kawamura et al. 1998; Obayashi et al. 1998), becomes

Table 3. Average physical properties of C180 cores in CrA.

Core no. (7V(H2)> Mass R AVcc i(H2) 21 2 1 3 3 (10 cm" ) (M#) (PC) (km s" ) (10 cm" )

Without star formation 1, 3-4, 6-8 4.4 4.3 0.12 0.54 9.8 With less-active star formation . 5 11.9 12.2 0.12 0.67 24.0 With active star formation 2 26.6 102.2 0.23 1.36 27.7 All cores 1-8 10.9 17.6 0.13 0.66 13.8 Downloaded from https://academic.oup.com/pasj/article/51/6/911/1467816 by guest on 30 September 2021

h ' (a) J (b)

[ ^S5 ^ ^ L—^^H 1 0.2 0.4 0.6 0.8 1 1.2 1.4 1.( 22 2 log[

5 5 iii 4 (c) 4 (d)J c 3^ c 3 " 8 2 8 2 J 1 1 §•• 1 0 ^_ -1.2 -0.8 log(R/pc) log (M/ M@)

1 11 11 11 1 4 ii ii— i i "ii ii i

(e) . 3 (f) 1 * o 1 H \ ill 0 3.6 3.8 4 4.2 4.4 4.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 3 log[n(H2)/cm ] log(Mv]r//W)

Fig. 5. Histograms of (a) the line width, (b) average H2 column density, (c) radius, (d) mass, (e) average H2 number density, and (f) ratio Mvjr/M of cores with and without star formation are shown. The shaded and hatched regions indicate the most active star-forming core (Core no. 2) and the other star-forming core (Core no. 5), respectively. The open regions indicate the cores without star formation.

ratio. R CrA cloud shows somewhat similar characteristics of To summarize, we can conclude that star-forming star-formation activity: the existence of the star clus clouds tend to have a high column density, as well as ter called 'coronet' (Taylor, Storey 1984) and the high a smaller Mw-lT/M ratio. SFE up to ~ 40% at the high-density region of the cloud (Harju et al. 1993). The physical properties, such as M 3.4. Comparison with the p Oph Cloud and (iV(H2)}, are similar in the two clouds: 90 M® and 22 -2 Among the star-forming regions in the solar neighbor 1.8 x 10 cm on the average in p Oph cloud, whereas 102 M® and 2.7 x 1022 cm-2 in R CrA cloud, respec hood, the p Oph molecular cloud is a unique object; 18 young stars are heavily clustered, and extremely active tively. These values are largest among the C 0 cores in star formation is taking place with a high star-formation the solar neighborhood (table 4). The shapes of the entire efficiency (SFE) of - 20% (Wilking, Lada 1983). The cloud also resemble each other; both clouds show a 'head- tail' structure. In the p Oph cloud, the 'head' is pointing

Table 4. Average physical properties of C180 cores in nearby star-forming regions.

Region (iV(H2)) Mass R AKomp Distance References* 21 2 1 (10 cm" ) (Mm) (pc) (km s" ) (pc)

pOph 17.6 90 0.24 0.87 160 1 Chal 11.3 32 0.22 0.81 140 2 CrA 10.9 18 0.13 0.66 130 3 Tail 6.9 23 0.23 0.49* 140 4 Oph-north 5.4 14 0.19 0.72 160 1 Chall 5.1 20 0.22 0.78 180 2 Downloaded from https://academic.oup.com/pasj/article/51/6/911/1467816 by guest on 30 September 2021 L 1333 4.3 9 0.16 0.92 180 5 Lup 4.1 10 0.17 0.90 150 6 Cha III 3.6 13 0.23 0.88 180 2

*Line width at the peak intensity position. f(l) Tachihara et al. 1999; (2) Mizuno et al. 1999; (3) this work; (4) Onishi et al. 1996, 1998; (5) Obayashi et al. 1998; (6) Hara et al. 1999. toward the Sco OB2 association, suggesting some interac University and Carnegie Institution of Washington. We tion with the OB association. As for the R CrA cloud, the greatly appreciate the hospitality of all staff members head points toward the Upper Centaurus-Lupus (UCL) of the Las Campanas Observatory of the Carnegie In association at ~ 140 pc (de Geus 1992), nearly the same stitution of Washington. We also acknowledge that this distance of the cloud, 130 pc. The physical connection of project could be realized by the contribution from many the R CrA cloud to the UCL OB association is suggested Japanese public donators and companies. Three of the by Harju et al. (1993). These signatures might show that authors (AM, HO, and YF) acknowledge financial sup an interaction with nearby OB associations is a necessary port from the scientist exchange program under bilateral condition for such active cluster formation. As a result agreement between JSPS (the Japan Society for the Pro of the interaction, dense cores are effectively compressed motion of Science) and CONICYT (the Chilean National and become massive, high-column-density cores, leading Commission for Scientific and Technical Research). This to the formation of bound clusters. work was financially supported in part by Grants-in-Aid for Scientific Research from JSPS (Nos. 10044076 and 4. Summary 11740125).

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