Draft version February 27, 2002
Preprint typeset using LATEX style emulateapj v. 25/04/01
LINEAR SEQUENCES OF STARLESS CORES AND YOUNG STELLAR OBJECTS IN THE EAGLE NEBULA
Naoya Fukuda 1, Tomoyuki Hanawa 2, AND Koji Sugitani 3
[email protected]
Draft version February 27, 2002 ABSTRACT We observed heads of two molecular pillars in the Eagle Nebula using the Nobeyama Millimeter Array with a spatial resolution of 3, 5, and 3 arcseconds in the 13 CO(J = 1 0 0) line, C18 O(J = 1 0 0) line and 2.7-mm continuum, respectively. We found 6 13 CO sub-clumps and 4 C18 O cores. The 13 CO clouds are elongated so as to have a head-tail structure, with the heads orientated towards the O star exciting M16. The elongation is likely to be due to radiation or wind from the O star. The cloud surface appears to be compressed, as indicated by strong 13 CO emission at the cloud rim. The shapes of the 13 CO clouds are quite similar to those of the dark cloud observed in the near infrared. Three out of the 4 C18 O cores are located within one of the 13 CO clouds. One of the 3 cores, located near the tip of the 13 CO cloud, is associated with a 2.7-mm continuum emission peak and is most likely to be a protostar. It is not associated with a known near infrared source. The other two cores are located further from the O star and are most likely to be starless cores. Thus these C18 O cores are aligned in order of age, with more-evolved objects closer to the O star. This linear sequence suggests propagation of star formation activity, i.e., sequential star formation, driven by the O star. A similar sequence of a YSO and a C18 O core was found in the other head of molecular pillar. Subject headings: ISM: clouds | ISM: individual(M16) | ISM: molecules | stars: formation 1.
tion stars (e.g., review by Elmegreen 1993). The idea of triggered star formation is supported by observations that newly formed stars and star-forming clouds are aligned in chronological order. The classical example is the alignment of the OB associations (e.g., Blaauw 1964). Another small-scale example is the alignment of YSOs in bright-rimmed clouds (Sugitani et al. 1995). The physical mechanism of triggered star formation has been examined over the past few decades. The radiative driven implosion (RDI) model is one of the most probable models for triggered star formation (e.g., Le och & Lazare 1994). The RDI model describes the evolutionary sequence from cloud collapse to the quasi-stationary cometary phase. To test the models, it is essential to search for gas condensations in molecular clouds irradiated by an O star. The present authors used the Nobeyama Millimeter Array (NMA) 4 to search for high-density gas in the pillars of the Eagle Nebula. The 13 CO and C18 O lines, and the 2.7-mm continuum emission were used as tracers of highdensity gases. The spatial resolution of these observations was a few arcseconds, which is almost twice that in previous work. The search focused on the heads of 51 and 52 , because these regions are luminous in the submillimeter continuum (White et al. 1999).
introduction
The Eagle Nebula M16 is an example of a clusterforming region, containing recently formed OB stars. M16 is an HII region excited by the stellar cluster NGC 6611, and is associated with the lamentary molecular clouds known as the \pillars", 51 , 52 and 53 (White et al. 1999). Hester et al. (1996) reported numerous evaporating gaseous globules (EGGs) on the surfaces of the pillars using the Hubble Space Telescope (HST). In addition, numerous young stellar objects (YSOs) in and around the pillars have been discovered by Sugitani et al. (2002), who made deep observations of the cloud in the near infrared (NIR). These facts indicate that stars have formed recently in the Eagle Nebula. The pillars in the Eagle Nebula have also been observed with radio telescopes. 12 CO molecular gas is associated with the dark clouds of the HST image (Pound 1998). White et al. (1999) discovered submillimeter sources at the heads of the three pillars and suggested that the heads are probably in the earliest stages of protostar formation. Unfortunately, these millimeter and submillimeter observations were limited to about 7 0 35 resolutions, corresponding to 0:07 0 0:35 pc at the distance of the Eagle Nebula, 2 kpc (Hillenbrand et al. 1993). However, as the radius of a typical dense core in Taurus is 0.23 pc in the C18 O line (e.g., Onishi et al. 1996), higher spatial resolution is required to accurately observe similar objects in the Eagle Nebula. It is widely believed that OB stars compress nearby small clouds and trigger the formation of next genera00
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2.
observations and data reduction
We observed the heads of 51 and 52 in M16 in the 13 CO(J = 1 0 0) and C18 O(J = 1 0 0) lines, and the
2.7-mm continuum, using the NMA. We used the D-array con guration on November 25 and 26, 2000, the AB-array
Japan Science and Technology Corporation; Department of Physics, Faculty of Science, Chiba University, Inage-ku, Chiba 263-8522, Japan Department of Astrophysics, School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan Institute of Natural Sciences, Nagoya City University, Mizuho-ku, Nagoya 467-8501, Japan The Nobeyama Radio Observatory is a branch of the National Astronomical Observatory, operated by the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
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Linear Sequences in the Eagle Nebula
con guration on February 9 and 10, 2001, and the C -array con guration on March 5 and 8, 2001. The FX spectrocorrelator was used for the 13 CO line, which provided a velocity resolution of 0:084 km s 1 . The UWBC spectrocorrelator was used for the 2.7-mm continuum and the C18 O line (velocity resolution 21:8 km s 1 ). We observed NRAO0530 every 20 minutes for phase and ux calibration. Observations of Uranus in November 2000 and Neptune in February and March 2001 were performed for NRAO0530 ux calibration. The eld of view was about 1 in diameter, corresponding roughly to the size of the head of a pillar. The centers of the eld of view were ( ; )1950 = (18h 16m 00s:5; 013 50 07 ); (18h 15m 58s:6; 013 51 05 ) for 51 and 52 , respectively. Image maps were processed with the AIPS package. For 51 , the synthesized beams were typically 3: 6 2 2: 3 (P. A. = 08: 7) for 13 CO, 6: 4 2 3: 7 (P. A. = 011: 6) for C18 O, and 4: 3 2 2: 8 (P. A. = 08: 0) for the 2.7-mm continuum. For 52 , the synthesized beams were typically 3: 7 2 2: 4 (P. A. = 05: 7) for 13 CO, 6: 5 2 3: 9 (P. A. = 015: 1) for C18O, and 3: 9 2 2: 5 (P. A. = 08: 1) for the 2.7mm continuum. The rms noise levels were 0.17 Jy/beam for the 13 CO line, 0.010 Jy/beam for the C18 O line, and 1:7 2 10 3 Jy/beam for the 2.7-mm continuum. 0
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3.
results and discussion
3.1. 13 CO main component, C18 O, and 2.7-mm Continuum Figures 1a and b summarize our observations of the heads of 51 and 52 . The contours denote the 13 CO(J =
1 0 0) line intensity of the main velocity component. The C18 O(J = 1 0 0) line intensity and the 2.7-mm continuum emission are denoted by color and gray scales, respectively. The background is the NIR image taken by Sugitani et al. (2002). The positional accuracy between submillimeter maps and NIR images is better than 1 . The properties are summarized in Table 1. The intensity has a systematic error of 30%. We estimated the mass using the following equations: Mcore = 80:1M (d=2 kpc)2 (Tex =20 K) exp[5:27(20 K=Tex )] [(X (C18 O)=1:6 2 10 7 )] 1 (F=1 Jy) (1v=44 km s 1 ) and Mpeak = F d2 =B (T = 20K), where = 0:1(=1012 Hz) g cm 2 using = 1:2. As shown in Figure 1a, both the 13 CO emission and NIR image reveal a dark cloud having a head-tail structure. The dark cloud has a main tail lobe extending away from the head, with two minor lobe to either side (northeast and southeast). As the head is directed toward the O5 star, it is suggested that the tail lobes are formed by radiation or stellar wind. The dark cloud extends further to the southeast in the HST image (Hester et al. 1996), indicating that a less dense cloud extends behind the tail. The 13 CO emission is intense at the cloud rim and has three local peaks refer to here as 13 CO sub-clumps. The most luminous sub-clump lies near the tip of 51 . The other two are located to the northeast and southeast of the most luminous one. The 13 CO emission is intense on the sides facing the O5 star, and particularly intense at the tip of the head. This supports the above suggestion that the shape of the 13 CO cloud is formed by radiation or wind from the O5 star. The 13 CO cloud contains dense C18 O cores labeled a, b, and c in Figure 1a. Core a is associated with the most lu00
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minous 13 CO sub-clump, while core b coincides with a less luminous 13 CO sub-clump. Core c is not associated with a 13 CO sub-clump, although it is located within the 13 CO cloud. Core a is the most luminous and is located near the tip of 51 . The arc-like shape of core a suggests compression at the cloud head. Cores b and c are not appreciably larger than the beam size and are not clearly resolved. It is thought that cores a and b may be embedded in the surface compressed layer of the 13 CO cloud because the 13 CO emission is most intense near the rim. The 2.7-mm continuum emission has two peaks near the tip of 51 , labeled x and y in Figures 1a and 2a. Both peaks are located within the broad submillimeter emission peak of White et al. (1999). Peak x partially overlaps the core a and is associated with weak emission extending to the N. Peak y coincides with a NIR source P1, which is likely to be a class I candidate (Sugitani et al. 2002). It is also associated with weak emission extending to the NW, which may coincide with the broad 8.69-GHz peak (White et al. 1999) and Br peaks (Allen et al. 1999). Peak x is likely to be due to dust emission from a pre-circumstellar envelope as it is associated with the C18 O core. Peak y is attributed to dust emission from a circumstellar envelope as it is associated with the class I candidate. Note that the younger objects are further from the O5 star. Peak y, a class I candidate, is close to the cloud head. Core a is just within the cloud and is most likely to be a protostar. Cores b and c are further from the cloud head and are not associated with any known near- or midinfrared sources (Pilbratt 1998). These objects are most likely to be starless cores. This arrangement suggests the propagation of star formation due to interaction with radiation or wind from the O5 star. As shown in Figure 1b, 52 is qualitatively similar to 51 . Again, the 13 CO cloud coincides with the NIR dark cloud and not with that seen in the HST image. The head of the dark cloud is orientated toward the O5 star. The 13 CO intensity of the 52 head is about half that of the 51 head. Due to the limited eld of view, we were unable to detect the tail of the 13 CO cloud clearly. The 13 CO cloud in the head of 52 also has three 13 CO sub-clumps. The most luminous sub-clump is located near the tip of 52 , while the other two are located near the northern and southern parts of the cloud. Here also, the 13 CO emission is generally stronger at the rim, indicative of a compressed surface layer. We also found a C18 O core (labeled d in Figure 1b) associated with the most luminous 13 CO sub-clump. The core is as large as 7 , i.e., slightly larger than the beam size. The 2.7-mm continuum has two peaks, labeled v and w in Figures 1b and 2b, near the head of 52 . Peak v coincides with a class I candidate, the NIR source T1 of Sugitani et al. (2002). Peak v is likely to be the circumstellar envelope of a YSO. 00
3.2. 13 CO Envelope As described in the previous subsection, it is suggested that 51 and 52 are elongated by radiation or wind from the O5 star. To study this possibility, we examined the blue- and red-shifted components in the 13 CO emission. Figures 3a and b show the blue- and red-shifted components of the 13 CO line for the heads of 51 and 52 . Both
Fukuda et al. blue- and red-shifted components are drawn with colored contours and overlaid on the maps of main component of the 13 CO (gray scale) and NIR images. The sample spectra are plotted in the same colors. The sample spectra for 51 are similar to those of White et al. (1999). The total masses in the velocity range of the spectra are estimated to be 26 M in 51 and 12 M in 52 . In 51 , both 13 CO side-components are elongated from west to east. The blueshifted component exhibits good positional agreement with the upper lobe of the NIR structure, and the redshifted component coincides with the main tail of the NIR structure. Although some peaks are found in the side components, they are coincident neither with the C18 O cores nor with the 2.7-mm continuum peaks. If the upper lobe is slightly further away from us and the main tail is slightly closer to us, then these blueand red-shifted components may be due to gas owing along the cloud surface. In 52 , the blueshifted component is strong in the southwestern part of the cloud while the redshifted component is strong on the northern side. Note that the redshifted component and NIR dark cloud have good positional agreement in the northeastern part of the cloud. Only the 13 CO sub-clump in the northeastern part coincides with an emission peak in the blue- and red-shifted components. If the gas is owing along the surface of the cloud, we would expect to see the blueshifted gas extending toward us. In short, the observed blue- and red-shifted emission is consistent with the interpretation that both 51 and 52 are elongated by wind and radiation from the O5 star. 3.3. Linear Sequence of Cores and YSOs As shown in the previous section, we have found several emission peaks in the C18 O line and the 2.7-mm continuum in the heads of 51 and 52 . These emission peaks are compact and most likely correspond to molecular cloud cores. Most are associated with less dense 13 CO subclumps. YSOs also exist in and near the pillars (Sugitani et al. 2002). These sources are arranged in order of age; the more evolved YSOs are closer to the O5 star whereas objects in an earlier stage of development are further away. This arrangement suggests propagation of star formation activity towards the southeast.
3
Assuming that star formation activity is propagating in 51 , it is possible to compute the propagation speed. 51 contains core b, core a, and the NIR source P1, with an average projected separation of 0:1 pc. These objects most likely correspond to a starless core, class 0 object, and a class I object, respectively. The age di erence will then be roughly 105 yr. The average separation and age di erence yield an apparent propagation speed of 1 km s 1 , which is a little slower than the shock propagation speed of 1.3 km s 1 (White et al. 1999). This propagation speed of star formation is reasonable if the O5 star is triggering star formation in 51 . Both the wing-like shape and density enhancements at the rim of the 13 CO cloud are similar to the early stage of collapse predicted by RDI models (Le och & Lazare 1994). The alignments of YSOs and cores do not t prediction by past RDI models because such models are restricted to the formation of one dense core near the center of an originally uniform cloud. This may be due to the lack of consideration of self-gravity in these early models. Models that take into account both RDI and self-gravity will be needed in order to show the propagation of star formation. A similar arrangement was also found in bright-rimmed clouds (Sugitani et al. 1995). NIR sources have been found near bright-rimmed clouds associated with IRAS point sources. The NIR sources are thought to be pre-mainsequence stars older than the IRAS point sources, and the NIR and IRAS sources are arranged sequentially from the rims in order of age. 0
0
We thank the sta members of the Nobeyama Radio Observatory for their warm hospitality during observations and data reduction, and thank R. Matsumoto and S. Miyaji for their encouragement. We also thank the anonymous referee for improving the manuscript. This study is nancially supported in part by a Grant-in-Aid for Scienti c Research on Priority Areas from the Ministry of Education, Science, Sports and Culture of Japan (No. 10147105) and under the \Research and Development Applying Advanced Computational Science and Technology" program of the Japan Science and Technology Corporation (ACT-JST).
REFERENCES Allen, L. E., Burton, M. G., Ryder, S. D., Ashley, M. C. B., & Storey, W. V. 1999, MNRAS, 304, 98 Blaauw, A. 1964, ARA&A, 2, 213 Elmegreen, B. G. 1993, in Star Formation in Stellar Systems, III
Onishi, T., Mizuno, A., Kawamura, A., Ogawa, H., & Fukui, Y. 1996, ApJ, 465, 815 Pilbratt, G.L., Altier, B., Blommaert, J. A. D. L., Fridlund, C. V. M., Tauber, J. A., & Kessler, M. F. 1998, A&A, 333, L9
Canary Island Winter School, ed. G. Tenorio-Tagle, M. Prieto, &
Pound, M. W. 1998, ApJ, 493, L113
F. Sanchez (Cambridge: Cambridge Univ. Press), 381
Sugitani, K., Tamura, M., & Ogura, K. 1995, ApJ, 455, L39
Hanawa, T., Yamamoto, S., & Hirahara, Y. 1994, ApJ, 420, 318 Hester, J. J. et al. 1996, AJ, 111, 2349 Hillenbrand L. A., Massey, P., Strom, S. E., Merrill K. M. 1993, AJ, 106, 1906 Hester, J. J. et al. 1996, AJ, 111, 2349 Le och, B. & Lazare , B. 1994, A&A, 289, 559
Sugitani, K., Tamura, M., Nakajima, Y., Nagashima, C., Nagayama, T., Nakaya, Y., Pickles, A. J., Nagata, T., Sato, S., Fukuda, N., & Ogura, K. 2002, ApJ, 565, L25 White G. J. et al. 1999, A&A, 342, 233
4
Linear Sequences in the Eagle Nebula
Table 1 Properties of C18 O cores and 2.7-mm continuum peaks ID core a . . . core b . . . core c . . . core d . . . peak x . . . peak y . . . peak v . . . peak w . . .
Intensity (Jy)
Radius ( )
Mass (M )
10 7 5 7 6 7 4 3
8.8 4.0 2.5 3.1 6.8 6.5 2.5 2.0
00
1:2 2 10 5:4 2 10 3:3 2 10 4:2 2 10 1:6 2 10 1:6 2 10 6:1 2 10 4:8 2 10
1 2 02 02 02 02 03 03 0 0
(mJy/beam)
(a) DECLINATION (B1950)
-13 49 55 50 00 05 10 15 20 25 30 35 18
50
40 35 30 0
60.0
59.0 (mJy/beam) 40
(b) -13 50 55 51 00 05 10 15 20 25 30 DECLINATION (B1950)
45
5
38 O5
36 34 32 30
18 15 60.0 59.0 58.0 57.0 RIGHT ASCENSION (B1950)
Fig. 1.| Maps of 13 CO(J = 1 - 0), C18 O and 2.7-mm continuum overlaid on NIR images (Sugitani et al. 2002) for the heads of (a) 51 13 CO is shown in the lower left corner in each panel. Contours denote the b 01 01 13 CO main component in the range (a) v integrated intensity of the LSR = 24:4 0 25:1 km s , and (b) vLSR = 22:1 0 22:8 km s . The lowest
and ( ) 52 . Field of view is denoted by the circle. Beam size for contour level is the 3 C
18 O
a
b
level of 0.3 Jy/beam. The contour intervals are ( ) 0.3 Jy/beam and ( ) 0.15 Jy/beam. The integrated intensity of
a
b
2 03
is shown by a color scale normalized to red maximum [55 mJy/beam in panel ( ) and 40 mJy/beam in panel ( )] and the gray minimum
(3 level of 30 mJy/beam). Gray areas denote the regions where the 2.7-mm continuum exceeds the 3 level of 5:1
10
Jy/beam.
Fukuda et al.
5
(a) DECLINATION (B1950)
-13 49 55 50 00 05
x
10 15
y P1
20 25 18 16 01.0 60.0 59.0 RIGHT ASCENSION (B1950)
(b) DECLINATION (B1950)
50 55 51 00
w
v
05 10
T1
15 20 25 18 15 59.0 58.0 57.0 RIGHT ASCENSION (B1950)
Fig. 2.| Maps of 2.7-mm continuum overlaid on the NIR images (Sugitani et al. 2002) for the heads of (a) 51 and (b)52 . Beam size for level of 5:1 2 1003 Jy/beam. Contour intervals 04 Jy/beam. Sources P1 and T1 are class I candidates (Sugitani et al. 2002). are 8:5 2 10
the 2.7-mm continuum is shown in the lower left corner. The lowest contour level is the 3
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Linear Sequences in the Eagle Nebula
2.0 -13 49 55 50 00 00 23.0 05 (km m/s) 10 15 20 25 30 35 18 16 02.0 01.0 60.0 59.0 RIGHT ASCENSION (B1950) DECLINATION (B1950)
(a)
DECLINATION (B1950)
(b)-13 50 50 55 51 00 05 10 15 20 25 30
1.0 00 23.0 21.0 v (km/s)
18 15 60.0 59.0 58.0 57.0 RIGHT ASCENSION (B1950) Fig. 3.| Channel maps of 13 CO(J = 1 0 0) overlaid on NIR images for (a) 51 and (b) 52 . Blue contours denote integrated intensity a vLSR = 23:3 0 24:4 km s01 and (b) vLSR = 21:0 0 22:1 km s01 . Red contours denote integrated intensity in the range (a) vLSR = 25:1 0 26:1 km s01 and (b) vLSR = 22:8 0 23:9 km s01 . Contour levels are (a) 0.3, 0.9, and 1.5 Jy/beam and (b) 0.3 and 0.6 Jy/beam.
in the range ( ) The
13 CO
panel.
main components shown in gure 1 are denoted by a grayscale for comparison.
Sample spectra are shown in the right of each
