MAPPING THE DISTRIBUTION OF ACETALDEHYDE TOWARDS ORION KL

Transcription

1 MAPPING THE DISTRIBUTION OF ACETALDEHYDE TOWARDS ORION KL Ryan A. Loomis 1, Anthony J. Remijan 2, and Joanna Corby 1 1 Department of Astronomy, University of Virginia, McCormick Rd, Charlottesville, VA National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA Abstract With the advent of new broadband spectral line interferometric observations, we can now begin to fully characterize the spectra and distribution of complex organic molecules that have been largely ignored since their original detections using single dish telescopes. First detected in 1973, acetaldehyde (CH3CHO) has been detected in numerous sources including TMC-1, Sgr B2, Orion KL, and OMC-1; yet its distribution within these sources is still not well known. Unlike a number of other molecules, acetaldehyde is not observed to be concentrated in hot core regions toward Sgr B2(N), but to have an extended distribution, a trait shared by other aldehydes (Chengalur and Kanekar, 2003). An extended distribution may indicate formation through gas phase ion molecule reactions, or that the distribution is a result of non-thermal processes liberating the molecule off grain surfaces, while a compact distribution may indicate warm grain surface chemistry with subsequent desorption by thermal processes. In this paper we present multiple transition maps of acetaldehyde toward Orion KL as evidence of an extended distribution of acetaldehyde, suggesting a formation chemistry similar to that of Sgr B2(N). In addition, spatial correlations to other molecules in the region are shown, suggesting common formation chemistry for some aldehydes. 1

2 1. Introduction First detected in 1973, acetaldehyde (CH3CHO) has been detected in numerous sources including TMC-1, Sgr B2, Orion KL, and OMC-1 (Gottlieb et al., 1973; Matthews et al., 1984; Johansson et al., 1984; Turner, 1989; Turner, 1991); yet its distribution within these sources is still not well known. As with many other organic species, the detections and distribution of acetaldehyde has been investigated to determine the nature of the physical environments of astronomical regions and possible formation mechanisms of complex molecules. However, unlike a number of these other molecules, acetaldehyde is not observed to be concentrated in hot core regions, but to have an extended distribution toward Sgr B2N (Chengalur and Kanekar, 2003). The extended distribution is also inferred for other aldehydes toward Sgr B2 including glycolaldehyde (Hollis et al., ApJ, 2001) and propynal. This lends credence to a common formation mechanism for aldehydes that would occur in cold environments. Although these results have been inferred by multiple observations of alydehydes toward Sgr B2, similar observations for aldehydes have not been as clear in other well observed sources such as Orion KL. Although acetaldehyde is detected in multiple single dish observations toward Orion KL (Turner 1989, 1991, see also the PRIMOS survey at interferometric observations of Orion have conflicting and incomplete data, resulting in inconclusive evidence for the distribution of acetaldehyde toward Orion. Using the BIMA array, Liu (2005) reported a distribution of acetaldehyde spatially coincident with the compact emission of formic acid (HCOOH), suggesting that acetaldehyde was released into the gas phase by grain mantle evaporation, similar to formic acid. Conflicting with these observations, Friedel (2008) reports no detection of acetaldehyde or formic acid in a λ=1mm survey of Orion KL using CARMA, and suggests that the distributions are very extended and as such resolved out. Furthermore, there is no apparent explanation for the discrepancies between the Friedel (2008) and Liu (2005) observations; the compact emission of both formic acid and acetaldehyde should have been detected by Friedel (2008) based on the Liu (2005) observations. It is also important to note that in both cases, a single transition was used for mapping. Recent broadband mapping projects using the ALMA Orion KL Band 6 Science Verification data suggest that single transition maps may not be indicative of true molecular distributions due to a number of effects including, but not limited to, density, excitation, temperature and chemistry (B. Harris, private communication). Furthermore, HIFI observations of the Orion KL region show no acetaldehyde emission from the HEXOS key science program, suggesting that acetaldehyde is concentrated in the hot core regions or rotationally cold (Neill, private communication). These disparate observations call into question the accuracy of the existing results for the detections of transitions and the interferometric mapping of acetaldehyde toward the Orion KL region. In addition to the discrepancies in interferometric mapping data for acetaldehyde, a 3mm survey of Orion KL and Sgr B2, Turner (1989,1981), observed much higher than predicted intensities for several relatively weak b-type transitions compared to the corresponding a-type transitions for a number of species, with acetaldehyde being a particularly clear example. These two types of transitions 2

3 differ in their selection rules for the Ka and Kc quantum numbers, and the strength of the transitions corresponds to the a and b dipole moments for the molecule respectively. The a dipole moment for acetaldehyde is 2.5 times larger than b, leading to stronger a-type transitions and weaker b-type transitions. Nummelin et al. (2000) observed the same phenomena as Turner (1989, 1991) in a 1mm survey of Sgr B2. Both authors suggest that this may be due to acetaldehyde having two distinct components; a well behaved cool, extended component and a compact component towards hot core regions that undergoes radiative pumping. A population inversion from this pumping would explain the much stronger than expected b-type emission. We have undertaken a dedicated campaign to image the distribution of multiple a- and b-type transitions of acetaldehyde at both high and low spatial resolution toward the Orion KL region in the λ=3mm frequency range to attempt to settle the discrepancies between these sets of data. In addition to resolving the discrepancies between previous observations, an accurate determination of the distribution of acetaldehyde in Orion will help determine the formation route of acetaldehyde. In this paper we present evidence of an extended distribution of acetaldehyde, as well as various distributions of other molecules in the region. Evidence is shown for compact b-type emission as well, supporting Nummelin and Turner s theory of multiple emission components. November and December 2012, and taken in the B array configuration during January and February Five tracks of ~4 hours each were taken in each array configuration, with a total time of ~20 hours for each array configuration. 16 antennas were used with a phase center pointing position of α(j2000) = 05 h 35 m 14 s.5 and δ(j2000) = , although there were several tracks where one antenna was out of the array for maintenance. In the E array configuration, projected baselines ranged from kλ ( m), while in B array configuration, projected baselines ranged from kλ ( m). As such, B array provides a smaller synthesized beam of approximately 2 x 1, while E array provides a synthesized beam of 7.3 x 6.1. The correlator was configured for both B and E array in the lower sideband to have 7 narrow band 62 MHz windows in 3 bit mode for spectral measurements, and one wideband 500 MHz window for continuum measurements. 7 narrow band and one wideband window were also configured in the upper sideband. The correlator configuration can be seen in table 1. For B array, Uranus was used as a flux calibrator, was used as a bandpass calibrator, and was used for gain calibrations. For E array, 3c84 was used as a flux and bandpass calibrator, and was used for gain calibrations. After the data were calibrated, they were continuum subtracted and imaged using the CLEAN routine of the CASA software package (CITATION). 2. Observations Observations were taken using the Combined Array for Research in Millimeter- Wave Astronomy (CARMA). Data were taken in the E array configuration during Window LSB Freq (MHz) USB Freq (MHz) BW (MHz)

4 Table 1. Correlator Setup 3. Results Although observations were taken with both E and B array configurations, only E array configurations have been fully reduced and imaged at this time. Due to maintenance issues with the CARMA data archive, the arrival of B array data was delayed significantly, and they are still being reduced and imaged. These images will be finished and published in the near future. 3.1 Continuum One wideband 500 MHz window in the upper sideband was used to map the continuum emission at 108 GHz from the Orion KL region. No molecular lines were detected within the passband, and the whole passband was imaged without continuum subtraction. The final image is shown in figure 1, with known sources labeled on the figure. As seen in the image, the Hot Core, IRc5, and IRc6 regions are all observed, although not resolved due to the spatial resolution of E array. B array observations, once reduced, will be able to resolve these sources, and possibly provide new information about their structure, or confirm previous observations of the structure of the region. Figure 1. Continuum emission towards Orion KL 3.2 Acetaldehyde (CH 3 CHO) Acetaldehyde was searched for in 6 narrow band spectral windows, with all but one transition observed. Figures 2-9 depict these transitions at their respective velocity peaks. Some velocity structure was seen, with a slight velocity gradient extending from the south-west to the north-east, but is not shown in this paper due to length restrictions. Notable is that there appear to be two acetaldehyde peaks, one toward the Compact Ridge, and another to the southwest of this. Emission extends up towards IRc5. These observations are in qualitative agreement with those of Liu (2005), showing an extended distribution of acetaldehyde with two peaks, although the south-west peak is seen more prominently. In addition, the single transition not observed was the weaker b-type transition, possibly indicating that it was too compact and weak to be detected by the E-array configuration. If this is the case, we may still expect to observe it in B-array data. It is important to note that the b-type transition observed does not follow the same distribution pattern as all the a-type transitions, with a single peak toward the Hot Core. 4

5 Figure , ,13 b-type transition of CH 3 CHO Figure ,2-4 3,1 A a-type transition of CH 3 CHO Figure ,5-4 0,4 A a-type transition of CH 3 CHO Figure ,5-4 0,4 E a-type transition of CH 3 CHO Figure ,6-5 1,5 A a-type transition of CH 3 CHO Figure ,6-5 1,5 E a-type transition of CH 3 CHO 5

6 Figure ,2-4 3,1 E a-type transition of CH 3 CHO Figure ,4-4 2,3 a-type transition of t-hcooh 3.4 Methyl Formate (HCOOCH 3 ) Figure ,3-4 3,2 A a-type transition of CH 3 CHO Four transitions of methyl formate were observed in one narrow band spectral window. These transitions mapped identically, so due to space constraints, only the transition is shown in figure 11. No velocity structure was observed in these images, and emission is clearly compact around IRc5. This is in qualitative agreement with the measurements of both Friedel (2008) and Liu (2005). 3.3 Formic Acid (t-hcooh) Three transitions of formic acid were observed in 2 narrow band spectral windows. These transitions mapped fairly similarly, so due to space constraints, only the transition is shown in figure 10. Prominent velocity structure was observed for these transitions, with the same velocity gradient extending from the south-west to the north-east. Once again, these observations are in agreement with those of Liu (2005). It is important to note, however, that formic acid does not appear to be cospatial with the south-western peak of acetaldehyde. Figure ,6-8 4,5 A a-type transition of HCOOCH Formamide (NH 2 CHO) Two transitions of formamide were observed in one narrow band spectral 6

7 window. These transitions mapped identically, so due to space constraints, only the transition is shown in figure 12. No velocity structure was observed in these images, and emission is clearly compact around the Hot Core. Although not observed in Friedel (2008) or Liu (2005), it is important to note that in both of those studies, all nitrogen bearing species were observed to mainly peak around the Hot Core, possibly explaining why formamide peaks here, rather than toward the Compact Ridge or IRc5 with other oxygen bearing species. extremely strong (> 5Jy/beam), and are most likely masing. This may explain why they map differently than the non-masing transitions measured in Friedel (2008) and Liu (2005). Figure ,2-1 0,1 a-type transition of CH 3 OH 4. Discussion 4.1 Distribution of CH 3 CHO Figure ,3-4 3,2 a-type transition of NH 2 CHO 3.6 Methanol (CH 3 OH) Three transitions of methanol were observed in one narrow band spectral window. These transitions mapped identically, so due to space constraints, only the transition is shown in figure 13. Slight velocity structure was evident, with the southwestern tail of emission being more prominent at lower velocities. The emission appears to peak around IRc5, extending to the south-west in a similar manner to acetaldehyde. Methanol was observed by both Friedel (2008) and Liu (2005), but they mapped it to be much more prominent toward the Hot Core. The observed transitions in this study, however, were From figures 2-9, it is clear that there are two peaks in acetaldehyde emission; one toward the Compact Ridge, and another to the south-west, with emission extending at lower levels up toward IRc5 and wrapping around to south of the Hot Core. These observations are similar to those of Liu (2005), but now provide multiple transition evidence of this distribution. As seen in figures 2-9, each transition has a slightly different distribution, possibly indicating multiple temperature components within the region. When B-array data is reduced, line profiles will be fit to all transitions and rotational temperature/column density calculations will be performed. Although the distribution of all a-type transitions is clearly extended, it is not yet clear as to whether there are additional compact components. If acetaldehyde emission is not observed in the B-array data, 7

8 it will be clear that there are no compact emission components. In agreement with Nummelin and Turner s predictions, however, is the distribution of the single observed b-type transition. It appears compact toward the Hot Core, with a different characteristic velocity (one more appropriate to the Hot Core rather than the Compact Ridge or IRc5). If this distribution is observed again in B-array data, it will provide strong evidence for two distinct components of acetaldehyde emission, with non-lte emission possibly caused by radiative pumping towards the Hot Core. 4.2 Other Molecular Distributions Observations of the distribution of formic acid are in qualitative agreement with those of Liu (2005). It is unclear why Friedel (2008) did not observe any formic acid emission, although B-array data may shed light on this, as the spatial resolution will but more similar to the Friedel observations. The compact distribution observed for methyl formate is in exact agreement with both the Friedel and Liu observations, and although neither previous study had observed formamide, it is reasonable to expect a compact distribution toward the Hot Core, where Liu (2005) had observed all nitrile emission. It is very interesting that the observed distribution of methanol emission is quite different from that seen in Liu (2005) or Friedel (2008). In particular, the distribution shown in figure 13 is clearly seen for multiple transitions in this study, but is not in agreement with the single transition maps of Liu and Friedel, which corroborate each other. As noted in section 3.6, this may be due to masing effects, as the methanol transitions we observed were extremely strong (>5 Jy/beam). B-array data will allow us to see if there are compact components to this emission, or if it is all extended. 4.3 Implications for Formation Routes Liu (2005) suggested a grain warm-up model for the formation of acetaldehyde, based on its spatial coincidence with the compact emission of formaldehyde. It is clear from the images in this study, however, that the southwestern peak of acetaldehyde is not spatially coincident with the compact formic acid emission, and that the acetaldehyde emission is much more extended. This may indicate formation through gas phase ion molecule reactions, or that the distribution is a result of nonthermal processes liberating the molecule off grain surfaces. For example, passing shocks through the region or external irradiation from UV or cosmic rays may be responsible. The presence of acetaldehyde b-type emission toward the Hot Core, however, suggests that acetaldehyde may be formed in compact regions, albeit at much lower column densities. If B-array data provides clear evidence for both b-type transitions toward the Hot Core, it may be possible to use a radiative pumping model to determine the column density in the region. 5. Conclusions We have observed 8 transitions of acetaldehyde towards Orion KL, and presented evidence for a widespread extended distribution in the region. These observations are in qualitative agreement with those of Liu (2005). However, a single b-type weak transition of acetaldehyde was observed to be compact toward the Hot Core region, suggesting possible masing due to 8

9 radiative pumping. Additional B-array data when reduced will either provide evidence of smaller compact components of emission, or additional evidence for only a widespread extended distribution. From this observed distribution, we conclude that a grain warm-up model is not sufficient to explain the abundance of acetaldehyde in the region, and that additional mechanisms, such as ionmolecule reactions or non-thermal grain liberation must be responsible for some of the observed emission. 6. Acknowledgements The authors gratefully acknowledge support from the Virginia Space Grant Consortium, the College Science Scholars program at the University of Virginia, and the Summer Student Research program at the National Radio Astronomy Observatory. We also thank B. McGuire for helpful comments on preparing observing scripts and using CARMA. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. CARMA operations and development are supported by the National Science Foundation and CARMA partner universities 3. Gottlieb et al., Molecules in the Galactic Environment. Wiley, New York, Hollis et al., ApJ, 554,L81-L85, Johansson et al., Astron.Astrophys., 130, , Liu, Astrochemistry Proceedings IAU Symposium, Matthews et al., ApJ, 290, , Nummelin et al., ApJSS, 128, , Turner, ApJSS, 76, , Turner, ApJSS, 70, , References 1. Chengalur and Kanekar, Astron. Astrophys., 404, L43-L46, Friedel et al., ApJ, 672, 962,