• Ei tuloksia

In this paper, our aim was to study the physical properties, morphology, kinematics and dynamics of the high-latitude dark cloud L 1155/1158 in Cepheus. Lynds 1155 is a dense core within the dark cloud L 1158, which is part of a cloud complex including L 1147/48, L 1152 and L 1157. This complex is located at the relatively high galactic latitude ofb∼15. The distance of L 1147/58 has been estimated to be 350 pc (Witt et al. 1982). In this cloud complex, star formation has recently occurred in the cloud L 1152. This has two IRAS sources, which according to their colours, are found to be Class I sources (Benson et al. 1988). In addition, at the North-Eastern edge of the cloud L 1158 there is a pre-main-sequence star, PV Cephei (Levreault 1984; Cohen et al. 1981), which has been extensively studied after the publication of our paper IV (Scarrot et al. 1991; Li et al. 1994). Goodman & Arce (2004) have very recently suggested that PV Cephei may have formed in, and been thrown out of, the massive star-forming cluster NGC 7023, more than 10 pc away from its present location.

For our purposes, we conducted CO, 13CO, C18O, HCO+ and NH3 observations and star counts in the direction of this cloud. Our C18O, HCO+and NH3observations showed that the cloud L 1155 actually consists of two separate cloud clumps which we called L 1155 C1 and L 1155 C2. The maxima of the H12CO+and NH3(1,1) radiation temperature maps coincide with each other, except in the front region between these clouds where a maximum is seen in the HCO+ emission but not in NH3. This third HCO+ emission maximum is probably due to decreasing self-absorption of HCO+ emission outside of the densest part of the cloud L 1155 C1. In the cloud L 1155 C1, the C18O, HCO+ and NH3 line velocities show a systematic pattern which indicates rotation of the cloud. The kinetic temperatures of the central regions are found to be around 10K. Using the NH3 observations, we found for the number densities of H2 values of 1.6×104 and 6.9×104 cm−3 at the centres of the clouds L 1155 C1 and L 1155 C2 respectively. The masses of the cores C1 and C2 inside the boundary of TR≥0.5K are approximately 14 and 4M respectively. According to the Jeans criterion, the core C1 might be gravitationally unstable. We found that the clouds L 1155 C1 and C2 could be a physically bound pair. It is possible to explain the presence of a bound pair of dense clumps L 1155 C1 and C2, embedded within the rotating extended cloud L 1158, with the fragmentation of the parent cloud.

Since the publication of Paper IV dates back to 1991, a review of the advances in the field is in order. Since that time, L 1155 has been included in several surveys of low-mass cores with and without stars (e.g. Williams et al. 1998; Caselli et al. 2002;

Walsh et al. 2004) and pre-stellar cores (e.g. Ward-Thompson et al. 1999, 2002;

Park et al. 2004). In the study of Ward-Thompson et al. (2002) using ISO data, no evidence was found for a central heating source in these pre-stellar cores. Yonekura et al. (1997) conducted a large-scale 13CO(J = 1–0) survey in the direction of the Cepheus and Cassiopeia regions, also including L 1155. They estimated the mass of the entire complex L 1148/L 1152/L 1155/L 1158 to be 1500 M.

Figure 2: On the left-hand side is the integrated intensity map of N2H+ emission and the CS profile map, and on the right-hand side averaged CS and N2H+ spectra in the direction of L1155 C2. The CS and N2H+ spectra are averaged within the half-maximum contour of the N2H+ emission. The (0,0) position is located at R.A.

20h43m00.0s, Dec. 67o41’47” (1950.0). The N2H+peak velocities have been indicated with the dashed lines. For the N2H+ integrated intensity, the lowest, half-maximum, and peak levels are 0.075, 0.374, and 0.748 K km s−1, respectively. (From Lee et al.

2001, reproduced by permission of the AAS.)

We concluded that the core L 1155 C1 might be collapsing. This view has received support from the studies of Lee et al. (1999, 2001). Based on the CS(2-1) and N2H+ (1-0) observations, they found that L 1155 C1 was an infall candidate. Using CS(2-1) and N2H+(1-0) observations, Lee et al. (1999) regarded L 1155 C2 as “a strong infall candidate”. This was confirmed by Lee et al. (2001). In Figs. 2 and 3 are shown on the left-hand sides the integrated intensity maps of N2H+emission and the CS profile maps and on the right-hand sides averaged CS and N2H+ spectra in the direction of L 1155 C2 and L 1155 C1, respectively. An extended infall asymmetry can be seen in the CS profile maps, and in the averaged spectra, the infall profiles are very clear.

This cloud was initially included in Mattila et al.’s long-range program of molecular line observations of high-latitude dark nebulae (Mattila et al. 1985). The author conducted additional observations of the cloud with Mets¨ahovi 14-m radio telescope.

She reduced all the molecular line radio observations from the object and performed

Figure 3: The same as in Fig. 2 but for L1155 C1. The lowest, half-maximum, and peak levels are 0.070, 0.348, and 0.695 K km s−1respectively, for the N2H+integrated intensity. Because of the large velocity gradient in this core, the systemic velocity of the averaged spectra have been shifted to coincide with that of the brightest N2H+ position. (From Lee et al. 2001, reproduced by permission of the AAS.)

the analysis. Important instructions were provided by the co-authors. The author wrote the article, but the co-authors assisted significantly.

7 Concluding remarks

The most intriguing feature of molecular clouds is that they are the sites of birth for stars. In addition, they contain about half of the mass of the ISM in the Galaxy and are also the coolest known objects in the universe. Thus, as well as being inter-esting objects for research by themselves, we also need to study their physical and chemical structure and their evolution in order to understand the formation of stars.

In this thesis I have presented investigations of some essential physical factors and processes which influence the mass estimation and are thought to be important for the morphology and evolution of interstellar molecular clouds.

The CO-to-H2 abundance ratio is a crucial quantity for estimating the masses and dynamical states of molecular clouds. In this thesis, observational evidence has

been presented from seven nearby dark clouds and globules, indicating that this ratio varies from cloud to cloud and even between the different parts of a unit cloud.

The variations are suggested to reflect the level of the star formation activity in the adjacent regions. We therefore conclude that caution should be taken when using the CO-to-H2 mass ratio, since no justification was found for the use of one canonical ratio for all clouds.

Some interstellar clouds consist of long, wavy threads of filaments. In such envi-ronments, the morphologies may indicate special arrangements of the magnetic field substructure in the clouds, and we wished to explore them by studying their polariza-tion patterns. A deep CCD polarimetry in the I band was carried out in selected fields in three filamentary, molecular clouds: the L 1400 complex, L 204, and MBM 25. Con-trary to expectations, the polarization patterns turned out to be remarkably smooth over the fields, particularly for the L 1400 complex, and there were no indication of any statistically significant difference in the degree or angle of the polarization be-tween obscured and non-obscured regions. We therefore concluded that the interstellar molecular filaments studied contribute very little to the interstellar polarization.

The morphology, kinematics and dynamics of dense interstellar clouds can often best be studied by means of molecular lines. Such a study of the dark molecular cloud L 1155/1158 in Cepheus was included in this thesis. Our C18O, HCO+ and NH3observations have shown that the cloud L 1155 actually consists of two separate cloud clumps, which we refer to as L 1155 C1 and L 1155 C2. According to the Jeans criterion, the core C1 might be gravitationally unstable. We also found that the cores C1 and C2 could be a physically bound pair, which is possible to explain with the fragmentation of the rotating parent cloud L 1158. Together with results from other complexes, these observations show that fragmentation of rotating clouds is a viable mechanism of core formation.

Interstellar molecular clouds, although a mature field of astrophysics, continue to exercise our minds. In particular, many open questions remain concerning their life-time and evolutionary history. Are molecular clouds supported against gravity or are they just like terrestrial clouds: dynamic and rapidly changing structures? Recently, it was proposed that molecular clouds are not long-lived objects as was assumed ear-lier. Such a scenario would remove the need for a mechanism supporting molecular clouds against self-gravity which, has been a central problem in many studies for some time now. For solving these puzzles, we need reliable methods for determining the essential physical parameters, such as masses of molecular clouds and magnetic field structures. Thus, the work done in this thesis has to be continued. Not just more data, but better quality data utilizing the latest observing techniques is needed in order to improve the statistics of this type of study and to gain a better picture of how CO behaves as an indicator of molecular gas under different cloud conditions, and also how polarization behaves, especially in quiescent clouds.

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