JYI: Variability of SiO Masers at 43 GHz in the Bipolar Nebula IRAS 19312+1950

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Issue 2, February 2004

Physical Sciences & Mathematics

Variability of SiO Masers at 43 GHz in the Bipolar Nebula IRAS 19312+1950

Kimberly Scott
Maria Mitchell Observatory and University of Evansville
Advisor: Vladimir Strelnitski, Ph.D.
Maria Mitchell Observatory

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Abstract

SiO masers at several transitions were detected in the bipolar nebula IRAS 19312+1950 by Nakashima and Deguchi in May 2000. While other SiO transitions show only one velocity component at approximately 26 km/s, the J = 1 0, v = 2 transition demonstrated another component at approximately 53 km/s. We observed the source in June 2002 in the J = 1 0, v = 1 and v = 2 transitions at 43 GHz with the 37 m telescope of the MIT Haystack Observatory at Westford, MA. In contrast with Nakashima and Deguchi’s result, the 53 km/s feature was detected in both transitions, but no feature was seen at 26 km/s. We conclude that this source shows two centers of strongly variable maser activity that are widely separated in radial velocity (approximately 27 km/s) and probably also widely separated in space. Further observations, in particular high-resolution interferometry, are needed to specify the geometry (quasi-spherical or disk-shaped) and kinematics (expansion, contraction, and/or rotation) of the circumstellar gas producing this double-peaked maser spectrum and to understand the evolutionary status of the central star (whether very old or very young).

Introduction

SiO masers (Microwave Amplification by Stimulated Emission of Radiation) originate in the dense gas surrounding young stars in HII regions and old stars in the Asymptotic Giant Branch (AGB) phase. HII regions are nebulae rich in ionized hydrogen where new stars form. The AGB phase occurs late in a star’s life, when its diameter swells due to the outflow of energy from shell helium fusion, and the star becomes a bright, red supergiant. For masers to form, the SiO molecules in the gas around these stars must first be forced into higher energy states by some pumping mechanism, and a population inversion (when more molecules are in the upper energy state than in the lower) must be achieved. Masing occurs when photons interact with these SiO molecules. If the energy of the incoming photon is the same as the difference between two energy states of the molecule, it will cause the molecule to drop to the lower energy state and release a photon with the same energy, phase, and direction of the incoming photon. By studying the spectrum of maser emission, we can learn much about the object with which it is associated.

Most of the known SiO masers are associated with evolved, Long Period Variable (LPV) stars, which vary their brightness on a known period. The SiO maser emission in an LPV arises relatively close to the pulsating photosphere of the star — somewhere between the photosphere and the point of dust formation in the expanding stellar wind (Boboltz and Marvel 2000). Shock waves produced by the non-linear pulsations within the star may play some role in the formation of gas condensations and maser pumping, although pumping can also be due to the infrared radiation of the star.

In LPV stars, masers are characterized by narrow (a few km/s) and apparently irregular spectral profiles close in radial velocity to the systemic velocity (the radial velocity of the central star). The SiO masers vary periodically in phase with the IR flux (Alcolea et al. 1999). SiO maser radiation from an LPV typically disappears soon after mass loss from the star ceases and it enters the post-AGB phase (Nyman et al. 1998). During its post-AGB phase, a star is surrounded by a Proto-Planetary Nebula (PPNe) that formed from the gas ejected from the star’s surface during the AGB phase. SiO masers in several PPNe surrounding post-AGB stars have been detected (Nyman et al. 1998; Nakashima and Deguchi 2000). Generally, the properties of SiO masers from PPNe are similar to those from other evolved objects (Nyman et al. 1998).

The prototype, and perhaps the only reliably identified SiO maser associated with a young star, is that in Orion IRc2, which is located in the Orion-KL nebula. The spectrum of the J = 1 0, v = 1 maser emission in this source (where J and v are the quantum numbers characterizing, respectively, the rotational and the vibrational energy of the molecule) is double-peaked and probably originates in a disk that is rotating and expanding about the young O or B star (Barvainis and Clemens 1984). The double-peaked profile of its spectrum distinguishes Orion IRc2 from other SiO sources. In this star, the two peaks are separated by about 20 km/s, with almost no emission in between (Alcolea et al. 1999). Originally, Barvainis attributed this to the rotating circumstellar disk being nearly edge-on in our line of sight, with the two peaks representing the approaching and receding edges of the disk. However, more recently, the possibility of more complex kinematics, including a bipolar outflow, has also been discussed (Greenhill et al. 1998). The pumping of the maser is thought to be provided by the IR radiation from the star. The maser flux had not shown noticeable variations in time within a decade after the discovery of the SiO maser in this source in 1974 (Barvainis 1984); however, from more recent observations, we know that the SiO maser emission from this source does in fact vary with time, but with no obvious regularity (Alcolea et al. 1999).

The object of this study, IRAS 19312+1950, was first classified as a post-AGB star surrounded by a bipolar nebulosity that is probably rotating and expanding (Nakashima and Deguchi 2000). A bipolar nebula forms when outflow from a star is directed primarily along the star’s polar axis. This SiO source is unique for an evolved star in that its spectrum for the J = 1 0, v = 2 transition exhibited two widely separated peaks when first detected in May 2000 (Nakashima and Deguchi 2000). This profile is reminiscent of the SiO maser emission from Orion IRc2 – a young star. Furthermore, IRAS 19312+1950 is much redder than other evolved stars with SiO maser emission. Its colors, C12 = log(F25/F12) = 0.50 and C23 = log(F60/F25) = 0.78 (where F12, F25, F60, are IRAS flux densities at 12, 25, and 60 microns), resemble those of a young star in a star-forming region (Nakashima and Deguchi 2000). Results of observations of this source to date make it difficult to classify this object as either a post-AGB star or a young stellar object. Few publications on maser observations of IRAS 19312+1950 exist, and little is known about the geometry and kinematics of the circumstellar dust and gas surrounding it. A careful study of the SiO maser emission can cast light on the nature of this unusual source. Here we report on new observations of the masing SiO lines in IRAS 19312+1950, which confirm the double-peaked profile of the maser spectrum and detected for the first time its strong variability.

Observations

IRAS 19312+1950 was observed in the J = 1 0, v = 1 and v = 2 SiO transitions at 43.1221 and 42.8206 GHz, respectively, in June 2002, with the 37 m telescope of the MIT Haystack Observatory at Westford, MA. The beamwidth of the antenna at these frequencies is close to 50”. The antenna aperture efficiency was 11%, which corresponds to a flux density to antenna temperature ratio of 22.8 Jy/K. The typical system temperature was between 170 K and 300 K.

The spectrometer, an autocorrelator, was operated with a bandwidth of 17.8 MHz (124 km/s) for both lines. This bandwidth was split into 2048 channels, giving a spectral resolution of 0.06 km/s before smoothing. Combining the results of two video converters, effective integration times of 34 hours for the J = 1 0, v = 1 transition and 10 hours for the v = 2 transition were achieved. The spectra were reduced and analyzed using the CLASS software downloaded from IRAM’s website.

Results

The results of the observations are summarized in Table 1, and the fitted spectra, smoothed to a resolution of 0.121 km/s, are shown in Figures 1 and 2. The signal-to-noise ratio is 3.3 and 2.1 for the J = 1 0, v = 1 and v = 2 lines, respectively. For both transitions, the spectrometer was centered at 26 km/s, where the main component was detected by Nakashima and Deguchi (2000) for both transitions, with an integral intensity of 1.8 and 1.1 Jy km/s, respectively. The precise values of the radial velocities measured by Nakashima and Deguchi (2000) for this component were 25.5 and 26.8 km/s for the two transitions, respectively. They also suspected a weaker (1.0 Jy km/s) second component at 52.8 km/s, but only for the J = 1 0, v = 2 transition.

Figure 1 . SiO maser spectrum of IRAS 19312+1950: J = 1 0, v = 1, 43.1221 GHz.

To our surprise, we did not detect any feature close to 26 km/s in either of the two transitions. However, we detected for both transitions a relatively strong (1.4 Jy km/s) feature near the second radial velocity. The measured radial velocities of the feature were 53.03 + 0.08 km/s for the J = 1 0, v = 1 transition and 51.64 + 0.12 km/s for the J = 1 0, v = 2 transition.

Figure 2. SiO maser spectrum of IRAS 19312+1950: J = 1 0, v = 2, 42.8206.

Table 1 . Results of SiO Maser Observations of IRAS 19312+1950.

Discussion

Our observations confirm the double-peaked shape of the SiO spectrum of IRAS 19312+1950, which was suspected from the observations of Nakashima and Deguchi (2000). Pronounced double-peaked spectrum with a large separation of the peaks is rare for SiO maser sources and it may be a clue to understanding the nature of the source.

Based on the bipolar appearance of the source from its Two-Micron All-Sky Survey image, Nakashima and Deguchi (2000) suggested that this source is a post-AGB star surrounded by a proto-planetary nebula. However, most of the SiO masers from evolved objects, including a few putative proto-planetary nebulae, show only a single feature centered at the source’s radial velocity. One exception has recently been found by Boboltz and Marvel (2000) in the SiO maser from NML Cygni. They detected a new peak, blueshifted by approximately 18 km/s from the one previously detected at about 0 km/s. Based on their VLA mapping of the source, Boboltz and Marvel (2000) explained the double-peaked SiO spectrum as a result of the rotation of a shell of SiO molecules surrounding the central star. Perhaps IRAS 19312+1950 is a second case of an evolved star with a rotating envelope capable of producing a double-peaked SiO spectrum.

Another possibility is that this source belongs to the even rarer case of a young star with SiO maser emission. In this case, a young star in IRAS 19312+1950 can be surrounded by a rotating/expanding disk - the interpretation first favored by Barvainis (1984) for the apparently young object with SiO maser emission in Orion-KL. However, it is difficult to reach a conclusion on this source based on observations to date. It would be interesting to see whether or not the SiO masers in this source vary with any periodicity. Also, interferometric mapping of this object would pinpoint the locations of SiO maser activity, and could aid in the classification of this object. To choose between the two interpretations (either an evolved star surrounded by a rotating shell of gas or a young star with a circumstellar disk), or perhaps to find a new one, careful monitoring of the variability and high-resolution mapping of this object is needed.

Conclusion

Our observations of the SiO maser in IRAS 19312+1950, when combined with the previous observations of Nakashima and Deguchi (2000), reveal the presence of strongly variable maser emission in two active velocity intervals, separated by ~27 km/s. This activity is revealed in two rotational lines of SiO. Monitoring of the variability and high-resolution mapping of this object are needed to choose between the current models of an expanding and rotating quasi-spherical envelope or a rotating and expanding circumstellar disk. Such observations will also help us to understand whether this source is an evolved star or a young star. Since only a few sources identified as either PPNe or young stellar objects have SiO maser emission, IRAS 19312+1950 is, in either case, a rare and interesting object. By studying this source in detail, we can learn much about stellar gas dynamics, either during star formation or at the end of the AGB phase.

Acknowledgements

The author would like to thank Vladimir Strelnitski for the opportunity to work on this project, and Richard Plotkin and the staff at the 37 m Haystack telescope for their help with the observations. This project was supported by the NSF/REU grant AST-0097694 and the Nantucket Maria Mitchell Association.

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References

Alcolea J et al. (1999) Six Years of Short-Spaced Monitoring of the v= 1 and v = 2, J = 1

0 SiO Maser Emission in Evolved Stars. Astronomy and Astrophysics Supplement Series. 139, 461-482.

Barvainis R. (1984) The Polarization of the SiO Masers in Orion: Maser Emission from a Rotating, Expanding Disk The Astrophysical Journal. 279, 358-362.

Barvainis R and Clemens DP. (1984) A Search for SiO Masers in Orion-Like Regions. The Astronomical Journal. 89, 1833-1835.

Boboltz DA and Marvel KB. (2000) Rotation in the Envelope of an Evolved Star: Observations of the SiO Masers toward NML Cygni. The Astrophysical Journal. 545, L149-L152.

Greenhill LJ et al. (1998) Coexisting Conical Bipolar and Equatorial Outflows from a High-Mass Protostar. Nature. 396, 650-653.

Nakashima J and Deguchi S. (2000) Detections of SiO and H2O Masers in the Bipolar Nebula IRAS 19312+1950. Astronomical Society of Japan. 52, L43-L46.

Nyman LA et al. (1998) SiO Masers in OH/IR Stars, Proto-Planetary and Planetary Nebulae. Astronomy and Astrophysics Supplement Series. 127, 185-200.

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JYI is supported by: The National Science Foundation, The Burroughs Wellcome Fund, Glaxo Wellcome Inc., Science Magazine, Science's Next Wave, Swarthmore College, Duke University, Georgetown University, and many others.