“I have already said that he must have gone to King’s Pyland or to Mapleton. He is not at King’s Pyland. Therefore he is at Mapleton. Let us take that as a working hypothesis and see what it leads us to. This part of the moor, as the inspector remarked, is very hard and dry. But it falls away towards Mapleton, and you can see from here that there is a long hollow over yonder, which must have been very wet on Monday night. If our supposition is correct, then the horse must have crossed that, and there is the point where we should look for his tracks.” – Sherlock Holmes, Silver Blaze
Table of Contents
- Footprints of star-formation
- Tracking back to the source
- Questions to be asked on this stage
- The main Aim of this thesis
Astrophysicists like criminologists are used to searching for “tracks” which could give them an important clues to uncover the “crime” – the astrophysical process. This usually starts with hypotheses and evolves towards the theory which is set to explain the supposition.
In one “case”, firstly observed in the 1950’s, small nebulae structures with similar spectra of nebulae around young stars (T-Tauri stars) have been uncovered by Herbig (1951) and Haro (1950, 1952). During further explorations it has been proposed that Herbig-Haro (HH) objects are the long searched for prestellar condensations (Herbig, 1965). But this hypotheses was undermined by further discoveries and analysis of proper motions of HH objects (Herbig & Jones, 1981; Jones & Herbig, 1982) showing that these objects have been ejected from nearby T-Tauri stars. Further exploration reveal that such behaviour is rather the rule than the exception and that stars with different luminosity have the same ability to “produce” HH objects (Cohen & Schwartz, 1983).
Another breakthrough in HH investigation has been made by works of Strom et al. (1972a,b) stating that HH objects can be found in the surroundings of very young objects such as Herbig Ae/Be stars, which have the age of 105 years, but they are absent in the surroundings of relatively old stars, such as T-associations around ρ Serpent (107 years). This significant finding has enabled astrophysicist to conclude that the lifetime of HH objects is less than the lifetime of T-Tauri stars and that HH object are connected with the early stages of stellar formation and evolution.
Further findings shaped the theory of formation and evolution and brought a deeper understanding of it. According to this star, in it’s earlier stages of formation and evolution, usually ejects powerful outflows which interact with the environment (surrounding molecular cloud material). This interaction leaves several distinct “footprints” or tracks, which can be recognized and used to track down the source – the protostar itself. Also they can be used to understand the physical properties and conditions of protostars.
The research case presented in the previous section suggests some clues which are set by nature, which may help to answer the long standing question: How do the star-formation processes leave their imprint on the environment?
It was already obvious before the HH-object detection that it is possible to point to the places where stars are forming by “looking” into them, because the process should leave some distinct marks which should give some answers (see Ambartsumian R.V., 1998, for a review). But it was only after the HH-object detection that the possibility to track-down the source appeared. The optical HH objects were already “pointing” to the direction of the possible source in the dark, obscured from optical wavelength, regions. This phenomena raised the question: Would it be possible to pin-point the source in the dark cloud? Later on, the radio observations, in the case of the HH 1/2 system, reveled the presence of the source which was deeply embedded in the circumstellar environment.
Figure 1.1: The HH 1 jet picture taken from Raines et al. (2000). The upper two panels show the clear advantage of the infrared imaging in comparison to the optical ones on the lower two panels.
The fact that HH objects are created by shock-exciting the media was realised later on from atomic spectroscopic study of the regions of HH objects. It was also realised that if there is enough force to shock-excite the interstellar atoms then it would also be enough power to shock-excite the molecules in the interstellar medium (ISM). But the main interstellar molecule was H2, and it was obvious that the radiation from that molecule should be possible to trace. The theory suggested the UV end and the IR could be the possible observing windows for that, both almost impossible to observe from the ground due to water vapour in the atmosphere. The alternative for observing the H2 excitation was to observe their rotational-vibrational excitation lines in the Near-Infrared. This was done consequently but the technique at that time (the end of 1970’s) was not good enough to image something. But the spectroscopical observations proved the presence of rotational-vibrational excitation lines in the HH objects. The significant contribution of the Near-Infrared started to appear only after the advent of a new generation of near-infrared CCDs, which were sensitive enough to detect and image the HH-objects in the Near-Infrared. Then because the Near-Infrared can probe deeper into the dark clouds, it was possible to track-down the IR counterparts of the HH objects. Figure 1.1 shows the recent result of the HH 1 jet observations in the optical and near-infrared from the HST. The astonishing fact is that the near-infrared can track down the outflow closer to the source than optical.
Figure 1.2: The HH 211 highly collimated bipolar jet located in the IC 348IR region. The jet has so far only been detected in molecular hydrogen near-infrared imaging by McCaughrean, Rayner, & Zinnecker (1994).
But this ability of tracking down the outflows closer to the source was not the only advantage of Near-Infrared imaging. The later observations of star-formation regions reveal the presence of many outflow structures which have no optical counterparts. Figure 1.2 displays the HH 211 outflow which was detected firstly in the molecular hydrogen near-infrared imaging by McCaughrean, Rayner, & Zinnecker (1994) and does not have any optical counterparts. This was suggesting that the outflow was so young that it still could not escape the cloud to be seen in the optical. Actually, the HH 211 outflow amongst the youngest ones, just over the 1000 years.
As like in the case of optical wavelengths, in the near-infrared the outflow is “pointing” towards the source, in the case of HH 211 very obviously. And the follow-ups at radio wave- lengths reveal the exact position of embedded outflow (Gueth & Guilloteau, 1999).
In the next step, optical wide-field surveys have “struck back” with the discovery of parsec scale outflows in the wider areas (e.g., the case of HH 46/47 Stanke et al. 1999). The advent of wide-field near-infrared cameras followed to enable the comprehensive search of H2 outflows in molecular complexes as well as in small dark clouds with high optical extinction.
The discovery of parsec scale jets changed our view of star-formation. How and why such jets could have emanated from the star were the crucial questions. The evidence that the stars can generate such outflows which can reach parsec scales started to speak about the power of the emanation, and consequently, about the power of the star-formation event. The models which were constructed to explain the observed jet origins started to estimate the mass of the star, the accretion rate etc.
This all shows that the study of the sites of interaction with the environment can spread light on the properties of the star-formation events. In this sense, tracking the H2 knots, which are the direct interaction places, can tell us about the star itself.
Now it is possible to understand how stars are forming without seeing them. The previous research tended to go closer and closer to the hypothetical source in order to reveal the mystery of the formation, but due to new evidence, we found ourselves searching further and further from the source.
There is more and more observational evidence in recent literature, which suggest that star-formation is not a stand-alone phenomenom. There are interactions with the environment during the whole process. In some sense, during the process of star-formation evolution, the environment also changes and interacts back, creating some interactive equilibrium or, rephrased, works in unison.
To test these preliminary thoughts, I specify some questions below, which might outline the particular directions where I would like to seek the answers in order to build the “case”.
-
Is there any isolated star-formation? This question is important in the sense that if isolated star-formation exists, then the models of star-formation can be well tested there. The reality is that the star-formation phenomena is widely observed and studied in the places where they firstly were found - in Molecular Clouds complexes such as Orion Molecular Cloud (OMC-I Stanke, 2000) and others, where the processes of formation and evolution can not be isolated. On the other hand there are small molecular clouds, such as Bok Globules (Bok & Reilly, 1947), which can indeed provide useful insights on isolated star-formation (Yun & Clemens, 1994a, 1995; Alves & Yun, 1995; Moreira & Yun, 1995; Khanzadyan et al., 2002a).
-
What is an outflow in general? This is a rather broad question and from the first glance it seems that there is sufficient scientific works done to explain the phenomena. But there is a need to explore the outflow structure in detail to constrain the new kind of models which will account for ISM chemistry and magnetic fields.
-
How does the outflow interact with the environment? The outflows do interact with the environment, there is no doubt in this. Otherwise we would not have HH and H2 objects, but how does this interaction responds back and changes the structure of the outflow? How does the magnetic field interact with the advancing flow and how crucial can that be for outflows? Can that change the shape and direction of the outflow?
-
Can the outflow change the course of star-formation? This is the question to which intuitively can be answered “YES”. But there is no clear evidence. It is logical to suppose that the outflows should change the environment and then the changed environment should change the course of star-formation, but how? The shocks from supernovae can change the environment dramatically, which can even cause a burst of star formation after all. But are the outflows from protostars powerful enough to change the environment so dramatically? There are outflows which are powerful enough to compress the material to high enough densities to enable the collapse of the media in the next 106 years or so. This can eventually cause the formation of one or a few stars - not much! But if we take one aggregate and suppose that the star-formation was going-on 106 years then if we take that in every 104 years we have an outflow event, there would be at least 100 outflows ejected through the medium. In small clouds this quantity can cause the disruption of the medium.
-
Is there any differences between the outflows in molecular clouds of different sizes? This remains a dilemma, because if the environment is “saturated” with outflows like in molecular cloud complexes, then it is hard to determine the real extent of the outflows. We are subject to selective effects. On the other hand, if we have some media where there are not many outflows around, it is probable to find and track down one or more outflows but it is hard to do statistics on that small number. Larger numbers of isolated sites are then needed. It might be possible that these two media cannot be compared at all due to possible different outflow emanation mechanisms.
The questions mentioned in the previous section have been tackled by many authors, but still there are no conclusive answers. This thesis cannot give conclusive answers to the questions either, but it would be reasonable to expect that this work will help and contribute towards our whole understanding of the situation with star formation environments. The main aim of this thesis is to follow the star-formation process in the sense of the interactions and influences which it has with the surrounding molecular material.
To tackle these issues I have selected to study isolated “laboratory” conditions such as Bok Globules. The reason is the strong desire to find out how the star or small aggregate of stars would evolve by interacting with the environment through outflows, and how these interactions would effect the formation processes in the globule, operating in unison.
I have selected a sample of close-by examples in order to derive the detailed characteristics of them. This would also enable me to understand the influence that outflows have on the environment’s chemistry and dynamics. All this information can then be scaled up for Bok Globules or scaled down for a very close star-formation region such as Rho Ophiuchus.
In the end, I expect to be able to derive some conclusive ideas for star formation which compares the different environments like Bok Globules, Rho Ophiuchus etc. For this purposes I have structured the thesis as follows:
- In Chapter 2, I will give an overview of Star Formation theory and observations which are applicable to the data and work of this thesis. I follow the path: Molecular Clouds → Protostars → the outflows.
- In Chapter 3, I will describe the observations of three different types of objects that have been carried out in the course of this study, which will be followed every time by the data reduction and analysis technique description.
- In Chapter 4, I will describe the results and discussion for the Bok Globules project.
- Chapter 5 will be dedicated to the results and discussion of the outflow project.
- In Chapter 6, I will describe the on-going project on Rho-Ophiuchus.
- Chapter 7 will be dedicated to the general discussion, which will bring together three described projects.
- And finally in Chapter 8, I will present the conclusions of this thesis followed by the prospects for future work.