Interview by Bo Reipurth, SFN #335 - November 2020
You started at Byurakan Observatory in Armenia as a student when Viktor Ambartsumian was leading the observatory. Did Ambartsumian spark your interest in young stars and star formation?
During my last year at Yerevan State University I was offered to work on long-slit photographic spectra of the famous NGC 2261 nebula (aka ``Hubble's Nebula) associated with R Mon, which drives the HH 39 flow. My advisor, Prof. Edward Khachikian, had been to USA and brought back to Byurakan a beautiful set of plates obtained by Jesse Greenstein at the 5-m Mt. Palomar telescope. I wrote my PhD thesis mainly based on this material. After that young stars and nebulae became my permanent interest. Khachikian introduced me already at the outset of my thesis work to Ambartsumian and in fact suggested him to be my advisor. However, as director of the observatory he was too busy, but did keep an eye on my work.
Ambartsumian was a brilliant theoretician, and made major contributions to early studies of star formation. As an observer, how much interactions did you have with him?
Ambartsumian did not do any observations himself, but he had a decisive influence on observational astronomy in Armenia by founding the Byurakan Observatory. In several ways, I became a professional astronomer at a very propitious time. At the end of 1975 both the Byurakan 2.6-m telescope (now known as the Ambartsumian telescope) and the 6-m telescope of the Special Astrophysical Observatory in northern Caucasus became operational. Almost immediately I started to do observations at these facilities: direct imaging (on plates, of course) on the 2.6-m and spectroscopy at the 6-m. In hindsight, I realize that it was Ambartsumian's authority that enabled me to start at such an early stage at these new large facilities.
Whenever I had obtained interesting new results I brought them to him for discussion. Ambartsumian liked to study and compare plates by himself. He was excited when, very soon after the start of observations at the 2.6 m telescope, the prominent variations of two cometary nebulae, PV Cep and V1515 Cyg, were discovered. Ambartsumian's greatest direct impact on me was his ability to point out some unusual phenomena and peculiar manifestations and then generalize them into new classes of astronomical objects. And even if afterward the initial explanations put forward turned out to be incomplete or even wrong, the objects themselves remained highly interesting and important for understanding the evolution of stars and galaxies. As examples I can mention the FU Ori type objects and the cometary nebulae. Ambartsumian remains the key influence in my work. Another important inspiration was George Herbig, whom I met at Byurakan Observatory and elsewhere.
In the 70s and 80s, when many star forming clouds were still largely unexplored, you focused on identifying very young, still partly embedded, stars through their reflection nebulae. These early papers were published in Armenian or Soviet journals that are mostly inaccessible in the West, so in 2003 you published your 'Merged Catalogue of Reflection Nebulae' in Astronomy & Astrophysics.
I had noticed that on the Palomar Observatory Sky Survey charts there are many small nebulous objects, which were not catalogued. I suggested to Armen Gyulbudaghian, who was finishing his PhD thesis, mainly on Herbig-Haro objects, a joint effort to prepare a list of all nebulous objects in dark clouds found (with a magnifying glass!) on the POSS charts. Ambartsumian encouraged us in this effort to identify new cometary nebulae and HH objects as signposts of the youngest stars. By the end of 1976 more than 100 nebulae and 36 probable HH objects had been identified, some of which we further studied at the 2.6-m and 6-m telescopes. To our surprise, several of the reflection nebulae showed short-term variability, indicating occulting material moving near their sources. Several other such lists were published around the same time or very soon after, notably those of Bernes and of Cohen (the well-known RNO list). In the process of doing these surveys I noticed that existing catalogs of nebulae contained ambiguous information and sometimes erroneous coordinates (measuring accurate coordinates in those days was not as quick and easy as today). So I collected all these scattered data into a merged list with accurate coordinates that became the 'Merged Catalogue of Reflection Nebulae' which has proven to be a useful resource. I later regretted that I did not include the RNO and our own GM objects in the list, which would have made it more complete.
You and your long-term collaborator Tigran Movsessian have for many years been interested in FUors. One particular region you have devoted much effort to is the little-known molecular cloud complex Lynds 1003, which harbors no less than two FUors.
All this started accidentally, while we were searching for new HH objects on narrow-band images from the 2.6-m telescope. The search was performed around small nebulous objects from various lists. This included the comparatively bright RNO 127 nebula, in the L1003 cloud in Cyg OB7. Our images identified it as a beautiful HH object. Unbeknownst to us, the region had already been observed by another group searching for emission-line stars, but their paper had not yet appeared. L1003 has proven to be very rich in young stars and outflow activity. We also noticed a nearby faint reflection nebula which during our monitoring increased in brightness, and a spectrum obtained with the 6-m telescope showed that it resulted from a rare new FUor eruption in an embedded star. We were lucky to be able to trace the further development of the nebula as the light echo played across the surrounding cloud, just as in classical cases. We also found that another nearby star, with an already known FUor-like spectrum, was surrounded by a very bright reflection nebula. Comparison with early sky survey images showed that this nebula appeared in the mid-1970s, thus defining the time of eruption. The most exceptional feature of the L1003 cloud is, in my opinion, not even the presence of two FUors near each other, but the fact that both of these recently erupting FUors are connected with rather long HH outflows. This is important supporting evidence for the idea that giant outflows are created during FUor outbursts, and that these outbursts indeed can be recurrent on timescales of several thousands years.
You and Movsessian have made major efforts in studying Herbig-Haro objects with imaging and spectroscopy using the Byurakan 2.6-m telescope. Please tell about the equipment and some of your results.
In 1984 Tigran Movsessian and I started to work together. We focused on the, then newly recognized, jet phenomenon from young stars using our 2.6-m telescope as well as the 6-m telescope, observations that delighted Ambartsumian.
In the late 1980s the situation in Armenia worsened, with a profound impact on Byurakan Observatory, where observations for several years stopped. Gradually, through international collaborations, we started up again. Especially fruitful was a collaboration with astronomers at Marseille Observatory who introduced the new methods of integral-field spectroscopy, and in 1996 helped to restart the observations on the 2.6-m telescope, by also providing a CCD detector and focal reducer for prime focus. We immediately started new searches for HH objects and jets, which soon were very successful. Movsessian also designed and constructed a separate multi-pupil spectrograph VAGR, and presently the 2.6-m telescope is equipped with a new multi-mode SCORPIO spectral camera. These various instruments have enabled us to study the internal structure and kinematics of HH objects and jets.
One of your most recent papers deals with the identification of a new star forming region in Canis Major. It is no longer very common to find such new regions. How did you find it, and what did you learn?
I first noted this clustering of nebulous objects in 1976, when compiling our first listings with Gyulbudaghian. But at that time it was really not possible to pursue even preliminary studies of this field, because of its -24 deg declination. But in 2003 during a visit to USA I suggested it as a promising field to John Bally, with whom we were already well acquainted, and who was able to obtain the necessary observations. This field indeed turns out to be a very active star-forming region, perhaps as rich as L1003, although not so immediately impressive because of its much greater distance. And so, at last, our common paper about it was published in MNRAS in 2016 - 40 years after first discovery!
What are your current projects?
During the last two years, we were able to completely rebuild the electronics and the control system of the classical 1 m Schmidt telescope at Byurakan Observatory. Russian astronomers from the Special Astrophysical Observatory have provided a modern CCD matrix and a large set of interference filters, as well as the data acquisition and reduction system. Thus, this famous telescope, one of the largest in the world of its kind, with which thousands of Markarian galaxies were discovered, again has become operational. Presently its field of view covers one square degree, but it could be enlarged in the future. The main observational program deals with the search for quasars and AGNs with z > 3. Several hundreds of new QSOs have already been found. We have ourselves started to observe some poorly studied star forming regions, and immediately had success in the Mon R1 association, finding there a lot of HH chains and compact reflection nebulae - yet another example of a strangely forgotten but promising field.
We plan to use data from this fine instrument together with invaluable archival resources such as Gaia, Spitzer, PanSTARRS, WISE and other data releases to gain a much more detailed picture of selected star forming regions. And it is my hope that new FUor and EXor eruptions will be discovered and studied, so we can develop a better classification of these important outburst phenomena.