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High resolution HCN and HNC spectroscopy in dark interstellar clouds
A.V. Lapinov
Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia

Introduction
Dense cores in dark clouds are associated with regions of low mass star formation. Despite of great interest paid to their studies by many authors detailed observations of these ob jects remain to be of great importance for physical and chemical evolution on early stages of star formation. The identi cation of such cores and determination of collapse and rotation velocity law in these regions are very important to test various models of star formation. On the other hand due to extremely low Tk only several K and nH2 = 103:::106 cm,3 dark clouds can be considered as unique physical laboratories" to study intermolecular collisions and spectroscopic molecular constants. The latter fact was demonstrated successfully in recent radio-astronomical spectroscopy of the J=1 0 hyper ne structure of HCNH+ Ziurys et al. 1992 and N2 H+ Caselly et al. 1995. It was established that due to di erences in excitation and radiative transfer of observed transitions and due to spatial abundance variations of observed species the information inferred from measurements of di erent molecules might re ect di erent physical conditions even in the same ob jects. From this point the good choice" of molecular lines to search and detailed studies of the collapsing cores play an important role in such investigations. It was found that collapsing phase of protostellar cores can be revealed by observations of asymmetric blue-bulge" lines of molecules with high critical densities at moderate line opacity. Nearly all surveys of protostar ob jects in dark clouds were made towards dense cores associated with embedded YSO identi ed originally from infrared measurements Gregersen et al. 1997, HCO+4 3&3 2; Mardones et al. 1997, H2 CO212 , 111 , CS2 1, N2H+1 0. The only example of as a starless collapsing core was L1544 which demonstrates a weak infall motion in the absence of any point like ob ject from IRAS measurements. It was revealed and studied extensively with single dish in CS2 1, N2 H+1 0, H2CO212 , 111, C3 H2221 , 101 by Tafalla et al. 1998 and observed with BIMA in N2H+ 1 0 Williams et al., 1999. In the presentwork we report additional collapsing starless cores revealed by us in recent high frequency resolution HCN measurements with IRAM-30m and Onsala-20m telescopes. Based on our observations we found that hyper ne split HCN line is a new good probe to search and to study collapsing cores in these regions. But because these measurements were proposed originally as a part of our program on HNC spectroscopy in dark clouds with extremely narrow line widths we start to consider our results with the HNC spectroscopy.

HNC spectroscopy in dark clouds

Because HNC molecule is very unstable in laboratory conditions and due to extremely narrow separation between hyper ne components, unresolved at room temperature, the


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only measurements of its hf structure were made by radio-astronomical spectroscopy. Despite of several attempts to determine the splitting of HNC J=1 0 transition Snyder et al. 1977, eQq=,0.4 MHz and Frerking et al. 1979, eQq=0:28 0:03 MHz the value of this splitting was known with a rather pure accuracy. This is mainly the result of a very small separation between hf components. The value of separation between the outer components is 200 kHz what is comparable with typical line width in cold interstellar clouds. This led to the fact that in all column density estimates of HNC molecules the J=1 0 transition is interpreted as unsplit even for dark clouds. As mentioned byTurner et al. 1997 the neglecting of hf splitting can lead to an underestimation of column density in observed clouds. We report the samples of observed HNC and HN13 C J=1 0 line spectra towards several dark clouds in Fig. 1. We have found that despite the measurements were made towards ob jects with extremely narrow line width 0:2 km s in HC3 N J=4 3 obtained byFuller and Myers, 1993 in most sources the observed pro les in main HNC lines are very wide due to optical depth broadening and complicated due to self-absorption in cloud envelopes see HNC pro le in L1512. Moreover we have found that in L134A the observed HNC line demonstrates anomalous hf ratio: F=1 1 is weaker than F=0 1, probably due to high self-absorption in optically thick F=2 1 and F=1 1 components. This anomalous structure explain the negative eQq value determined in this cloud bySnyder et al. 1977. From Fig. 1 it is seen that in B217SW even HN13 C J=1 0 is to broad to be used for estimates of hf splitting. But the quality of HN13 C lines in L1512 and L1498 and HNC line in CB4 is good for the purpose of spectroscopy. We report the results of estimates of HNC spectroscopic constants in Table 1 with the most reliable quantities marked as bold values. Table 1: HNC spectroscopic constant estimates Source L1512 L1498 V
LS R

eQq

CN VLS

CN L1400K VLS C CB4
N

eQq

R

eQq

R

V C

eQq

LS R

N

3 equal 3 individual 3 hf, LTE, and line widths line widths ,broadening 13 C HN 7.29987120 7.28002142 7.29195115 268.42154 275.71136 265.33129 4.1725 4.7131 5.7725 7.99697146 7.96519164 7.98579148 271.49174 284.52158 266.72159 3.5228 5.0936 5.8932 3.46076425 3.48834517 3.43677567 275.77507 263.49575 270.04614 6.2787 5.06113 11.12124 HNC -11.34161217 -11.31639263 -11.34466232 285.29252 268.09336 281.51271 0.1741 0.2460 1.2953


Interstellar matter. Galaxies Wehave found that despite the determined by us eQq=265...285 kHz agrees byFrerking et al., 1979 28030 kHz there is systematic shift 50 kHz or Table 2 between the rest HN13 C frequencies determined byFrerking et al. estimated from L1512 measurements for which the correct VLSR value was simultaneous C18 O2 1 measurements. The precise C18 O2 1 laboratory provided us by F.Lewen private communication Table 2: HN13 C J=1 0 hyper ne frequencies Component F=0 1 F=2 1 F=1 1 Frequency MHz Frerking et al.,1979 this work 87090.73546 87090.6731 87090.85946 87090.8111 87090.94246 87090.8841

83 with estimates 0.17 km s, see and our values determined by frequency was

HCN spectroscopy in dark clouds
Despite originally the high frequency spectroscopy of HCN J=1 0 was proposed only for measurements of correct LSR velocity of dark clouds in a frame of our HNC program, we have found that in all observed by us starless cores the corresponding HCN lines are optically thick and have asymmetric pro les with deep self-absorption see Fig. 2. Previously such kind of asymmetric HCN lines were predicted by us Lapinov, 1989 during study of anomalous hf ratio of HCN in dark clouds. The example of expected HCN line in collapsing dark cloud is shown in the Fig. 2 top. Due to di erent opacityin each hf component there is di erent in uence of absorbing envelope on the line shape. As a result each hf component could re ect di erent region on the same line of site what allows to consider HCN more e cient in comparison with molecular lines without hf splitting. In N2H+ 1 0 the hf splitting is more reach than in HCN but in most dark clouds N2H+ lines are optically thin see Tafalla et al. 1998 and Benson et al. 1998 what gives it less e cient in a search of collapsing cores in comparison with HCN molecule. The fact that in all observed by us starless dark clouds HCN F=2 1 and F=1 1 lines are self-reversed gives HCN molecule a good probe to search for collapsing cores in dark clouds. Note, that whereas in center positions of B217SW and L1498 the line asymmetry corresponds to inward motions, in L1512 this asymmetry is opposite. After IRAM measurements of self-reversed HCN lines we made an HCN map of B217SW in January 1999 with Onsala20m telescope. We found that despite the observed region has no revealed infrared ob ject except HH31 20 north-east from the cloud center this cloud clearly demonstrate bipolar structure with blue-bulge asymmetry type towards the center and SW blue lobe but redbulge asymmetry in the NE red lobe Fig. 3. It is seen that the observed bipolarity is more evident after the MEM deconvolution. In a present report we are going to compare performed by us HCN observations with available data from other measurements and to discuss in details results of the present data interpretation.
We greatly thank C.Thum from IRAM, L.E.B.Johansson and P.Bergman from OSO for their help in these observation. Additionally we thank F.Lewen from Koln University who provide us results of precise C18 O2 1 frequency measurements prior his publication.


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Figure 1: HN13 C and HNC J=1 0 line observed towards starless dark clouds with IRAM-30m 10 kHz resolution and Onsala-20m 12.5 kHz resolution radiotelescopes


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Figure 2: Calculated top and observed HCN J=1 0 spectra towards collapsing starless cores. For three ob jects we report also J=2 1 lines in CO isotopes


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Figure 3: B217SW observed and MEM deconvolved maps in HCN J=1 0
This research was supported by the INTAS grant 93-2168-Ext. and partially by grant 96-0216472 from the Russian Foundation for Basic Research.

References
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