ABSTRACT
We present results from a search for water maser emission toward N4A, N190, and N206, three regions of massive star formation in the Large Magellanic Cloud (LMC). Four water masers were detected; two toward N4A, and two toward N190. In the latter region, no previously known maser emission has been reported. Future studies of maser proper motion to determine the galactic dynamics of the LMC will benefit from the independent data points the new masers in N190 provide. Two of these masers are associated with previously identified massive young stellar objects (YSOs), which strongly supports the authenticity of the classification. We argue that the other two masers identify previously unknown YSOs. No masers were detected toward N206, but it does host a newly discovered 22 GHz continuum source, also associated with a massive YSO. We suggest that future surveys for water maser emission in the LMC be targeted toward the more luminous, massive YSOs.
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1. INTRODUCTION
The physical, chemical, and dynamical characteristics of galaxies are generally driven by the intense stellar winds and explosive deaths of high-mass stars within them. Observing these stars in different environments and in various stages of evolution leads to an understanding of the life cycle of these stars and their influence on galactic dynamics. The most poorly understood time of a massive star's life is the early accretionary stage as a young stellar object (YSO; Zinnecker & Yorke 2007). The puzzle of massive star formation is pieced together as these massive stars are found in the early stages of formation as YSOs.
Much of the difficulty in understanding massive star formation lies in finding massive YSOs. This is primarily due to their scarcity, large distances, rapid evolution, and dense, dusty environments. One of the best ways to study star formation is by observing molecular masers around YSOs (e.g., Goddi et al. 2011). The bright, compact radio emission emanating from the masers can be observed at great distances, even through the obscuring dust clouds that hide the environments of the YSOs at visible wavelengths. With the resolution provided by radio interferometry, it is possible to associate masers with individual YSOs, even at the distance of the Large Magellanic Cloud (LMC; Ellingsen et al. 2010). The 22.2 GHz masing transition of water is particularly useful, because it is the strongest transition among known maser species. Water masers are often associated with the outflows of high-mass stars (e.g., see Moscadelli et al. 2000) and thus often indicate the presence of massive YSOs (Beuther et al. 2002), although it is not uncommon for them to be found in the environments of low-mass YSOs (Wilking et al. 1994).
The study of massive star formation in the LMC is particularly useful because many of its physical characteristics differ from those of the Milky Way. For example, the LMC has less mass, a lower star formation rate, no spiral arms, a lower metallicity, and a greater ambient UV flux (Meixner et al. 2006 and references therein), all of which influence star formation on a galactic scale. Previous searches for water masers in the LMC have yielded relatively few detections. The recent paper by Imai et al. (2013) totals the number of detected LMC interstellar water masers at 23 among 14 regions. Green et al. (2008) suggest that the apparent underabundance of water masers in the LMC might be related to the its lower star formation rate (Israel 1980). They also note the difficulty in obtaining a sensitive, full scale survey for water masers in the entire LMC. Searches for these water masers may be more successful if they target known massive YSOs. Gruendl & Chu (2009, hereafter GC09) used infrared data from Spitzer's Surveying the Agents of a Galaxy's Evolution (SAGE) program (Meixner et al. 2006) to identify 1385 intermediate- or high-mass YSO candidates based on infrared colors and spectral energy distributions. Of those YSO candidates, they classify 855 as "definite," indicating a high degree of confidence in their nature. Ellingsen et al. (2010) found a high correlation between detected water masers and definite YSO candidates identified by GC09. In fact, 17 of the 23 interstellar masers discovered in the LMC as listed by Imai et al. (2013) are separated by less than 5'' of a GC09 definite YSO. An associated maser with a YSO candidate strongly supports the classification. Therefore, observations toward GC09 YSO candidates may be useful in finding these rare masers and thereby a better understanding of what the presence of water masers tells us about the physical characteristics of YSOs.
We report on observations in search of water masers toward three regions of massive star formation in the LMC. Details on observations and data reduction are found in Section 2. The results of the observations are reported in Section 3, with an analysis of those results given in Section 4. A summary of the results and conclusions is presented in Section 5.
2. OBSERVATIONS
Three regions of massive star formation in the LMC (N4A, N190, and N206; Henize 1956) were observed for the 22.2 GHz water maser line with the Australia Telescope Compact Array (ATCA). Their designations and the pointing positions are listed in Table 1. The ATCA is the radio interferometer in the southern hemisphere with the highest sensitivity and greatest resolution, thus the prime instrument to study the LMC.
Table 1. Targeted Regions
| Namea | R.A. (J2000) | Decl. (J2000) | Observing Date |
|---|---|---|---|
| (h m s) | (° ' '') | ||
| N4A | 4 52 09.1 | −66 55 14.8 | 2008 Mar |
| N190 | 5 04 25.4 | −70 43 49.2 | 2008 Mar |
| N206 | 5 30 23.5 | −71 07 04.8 | 2013 Feb |
| N206 | 5 30 48.0 | −71 07 55.2 | 2013 Feb |
| N206 | 5 31 00.7 | −71 06 54.0 | 2013 Feb |
| N206 | 5 31 22.3 | −71 06 54.0 | 2013 Feb |
| N206 | 5 31 22.7 | −71 04 11.6 | 2008 Mar |
Notes. The pointing centers for each of the regions of interest. aDesignation from Henize (1956).
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Archival data were acquired from the Australia Telescope Online Archive. The observation for this data was made on 2008 March 15 (part of program CX139). The ATCA was in its 1.5D configuration, observing at a central frequency of 22.215 GHz with a bandwidth of 16 MHz divided into 512 channels. This gives a spectral channel spacing of 31.25 kHz, yielding a velocity resolution of 0.422 km s−1. The integration time on each source was around 20 minutes over the course of 7 hr.
Additional observations were conducted with the ATCA on 2013 February 9 under program C2773. These observations investigated the N206 region in greater detail. The new data covered four additional pointings in N206 with the 6A array configuration. These observations were taken in an attempt to find water masers associated with 11 definite YSO candidates from GC09, and covered an additional 12.5 arcmin2 of emission from N206 for a total of ∼15.5 arcmin2. These observations were taken with two overlapping 2 GHz continuum intermediate frequencies (IFs) giving a total bandwidth of 3 GHz at a central frequency of 22.200 GHz. Since water masers often trace high velocity outflows from YSOs, a wide velocity coverage was necessary. To accomplish this, we utilized the ATCA's capacity to concatenate 16 1 MHz spectral zoom bands in 2 IFs and overlapped them to obtain a total spectral bandwidth of 15.5 MHz, giving a total velocity coverage of ∼210 km s−1 around the systemic velocity, sufficient to observe the high-velocity masers. The observations had a high spectral resolution of 0.488 kHz, or a velocity channel spacing of 0.007 km s−1. We then smoothed the data to a velocity resolution of 0.224 km s−1 to increase sensitivity. The integration time toward each pointing was 60 minutes over the course of 6 hr.
Both new and archival data were reduced using standard procedures in miriad. PKS B1934−638 was used as the primary amplitude calibrator and had a flux of 0.84 Jy. The flux scale has been determined to an accuracy of ∼10%. One major exception is that for the 2013 data, the primary flux calibrator for the ATCA (PKS B1934−638) was not observable during the course of the observation. Flux calibration was thus done on a secondary calibrator, PKS B0537−441. Based on observations over the past 2 yr, its flux in the K band has had an average value of 7.5 Jy, but has varied by ∼25%. We expect a similar error on our amplitude calibration for that data set. PKS B0637−752 was used as the phase calibrator in both experiments, and was observed once every ∼30 minutes for the 2008 data and ∼12 minutes in 2013. The phase calibrator was also used to update the pointing solutions about once an hour. The secondary calibrator flux was bootstrapped from the primary and the amplitude, phase, and bandpass solutions were applied to the program sources.
Images were made and deconvolved using the CLEAN algorithm using naturally weighted visibilities in order to maximize point-source sensitivity. Image sensitivity for the 2008 observations was ∼7 mJy beam−1 rms for N4A and N190, and ∼8.6 mJy beam−1 rms for N206. Fortunately, the signal-to-noise ratio for all detected masers in this work is very high. Image sensitivity for the 2013 observations was ∼20 mJy beam−1 rms and ∼82 μJy beam−1 rms in the maser and continuum images, respectively. Maser positions and peak flux densities were determined by the miriad task imfit, which fits two-dimensional Gaussians to the sources. The 2008 1.5D array data had a resolution of about 1
0 while the 2013 6A array had a resolution of about 05.
3. RESULTS
A total of four masers toward two high-mass star-forming regions were detected. Four maser spots were detected toward N4A and two were detected toward N190. Their positions, properties, and any associated GC09 YSO or Two Micron All Sky Survey (2MASS) point source (Cutri et al. 2003) are reported in Table 2. We judge that a maser is associated with a YSO if it is located within 2'' of the YSO (i.e., the resolution of both the Spitzer SAGE survey used by GC09 and 2MASS) after the practice of Ellingsen et al. (2010).
Table 2. Detected H2O Masers
| Source | R.A. (J2000) | Decl. (J2000) | Smaxa | Velocity | ΔV (FWHM) | Associations | Separation |
|---|---|---|---|---|---|---|---|
| (h m s) | (° ' '') | (mJy beam−1) | (km s−1) | (km s−1) | ('') | ||
| N4A | 04 52 05.24 | −66 55 13.9 | 230 ± 15 | 262.5 | 1.0 | J045205.39−665513.8b | 0.9 |
| 04 52 09.05 | −66 55 22.5 | 261 ± 10 | 259.9 | 1.0 | J045209.22−665521.9c,d | 1.2 | |
| 04 52 09.07 | −66 55 22.6 | 431 ± 30 | 258.4 | 1.0 | J045209.22−665521.9c,d | 1.1 | |
| 04 52 09.21 | −66 55 22.2 | 275 ± 24 | 272.5 | 1.1 | J045209.22−665521.9c,d | 0.3 | |
| N190 | 05 04 24.98 | −70 43 42.9 | 1271 ± 132 | 231.5 | 1.3 | J050424.82−704343.7c | 1.1 |
| J05042500−7043444e | 1.4 | ||||||
| 05 04 25.44 | −70 43 41.6 | 1615 ± 159 | 231.5 | 1.3 | J05042559−7043434e | 2.0 |
Notes. Masers detected in this work. The second and third columns list the positions of the individual maser spots. The fifth and sixth columns give the peak velocity and the FWHM of the best-fit Gaussian to the feature. The last two columns list the relevant associations and the separation of the maser spots from those sources, respectively. aPeak intensity. bGruendl & Chu (2009) diffuse source. cGruendl & Chu (2009) massive YSO. dContursi et al. (2007) Star 2. eCutri et al. (2003) 2MASS point source.
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N4A. Three of the four detected maser spots are within 12 of a definite massive YSO candidate identified by GC09 and spectroscopically confirmed by Seale et al. (2009). The other is associated with a region that GC09 and Seale et al. (2009) identify as a "diffuse" source, likely a local enhancement within a filament. The locations of the detected masers relative to the Spitzer Infrared Array Camera (IRAC; Fazio et al. 2004) 8.0 μm image are shown in Figure 1. The integrated flux of the central portion of both masers as a function of velocity are displayed in Figure 2.
Figure 1. IRAC 8.0 μm image of N4A. The massive YSO identified by GC09 and Contursi et al. (2007) is indicated with a cross. The positions of maser spots detected in this work are shown as white dots. The filamentary structure with the associated maser can be seen as an arc extending to the right of the brightest emission in the region.
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Figure 2. Spectra of the 22.2 GHz water masers toward N4A. Top: the three peaks of maser spots associated with the GC09 massive YSO. Bottom: the maser spot associated with the diffuse filamentary emission. Note that the two small peaks around 283 km s−1 are spurious signals due to an incomplete sampling of the uv plane.
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Standard image High-resolution imageN190. One of the detected masers is located within 1'' of a GC09 definite massive YSO, also confirmed spectroscopically by Seale et al. (2009). The other maser is farther away, separated by about 4'' from the massive YSO, and thus is not considered to be associated with it. The positions of the masers and associated 2MASS point sources are shown against IRAC 4.5 μm continuum contours in Figure 3. The integrated flux of the central portion of both masers as a function of velocity are displayed in Figure 4.
Figure 3. IRAC 4.5 μm contours of N190. Crosses indicate the positions and uncertainties of 2MASS J05042500−7043444 and 2MASS J05042559−7043434. The circles indicate the positions and uncertainties of the newly detected masers. The contours (10%, 15%, 21%, 24%, 27%, 30%, 40%, 60%, and 80% of the peak intensity) have a peak at the GC09 massive YSO, but they do show some emission associated with 2MASS J05042559−7043434 near the other maser.
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Figure 4. Spectra of the 22.2 GHz water masers toward N190. Top: the maser associated with 2MASS J05042500−7043444. Bottom: the maser associated with 2MASS J05042559−7043434.
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Standard image High-resolution imageN206. Although no masers were found, a 22 GHz continuum source was detected. Its position and peak flux are reported in Table 3. It is associated with yet another GC09 definite massive YSO confirmed by Seale et al. (2009). Despite the uncertainty in the absolute flux calibration, the 5.5σ signal and association with a known massive YSO indicate that it is a bona fide detection. The continuum source position and contours and its relation to the GC09 massive YSO is shown in Figure 5.
Figure 5. 22.2 GHz contours (at 220, 320, and 420 μJy beam−1) of the continuum source toward N206. The cross indicates the GC09 YSO position and its uncertainty.
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Table 3. Detected Continuum Source
| Source | R.A. (J2000) | Decl. (J2000) | Smaxa | rms | Sintb | Association | Separation |
|---|---|---|---|---|---|---|---|
| (h m s) | (° ' '') | (μJy beam−1) | (μJy) | ('') | |||
| N206 | 05 30 20.16 | −71 07 50.3 | 450 | 82 | 636 | J053020.33−710748.9c | 1.6 |
Notes. Position and flux of the 22 GHz continuum source detected toward N206. The last two columns list the associated massive YSO candidate and its separation from the continuum source, respectively. aPeak intensity. bTotal integrated flux. cGruendl & Chu (2009) massive YSO.
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4. DISCUSSION
4.1. N4A
N4A is the brightest region in the N4 H ii complex. N4A is about 210 in angular size (about 5.0 pc at the 50 kpc distance of the LMC) with two primary ionizing stars (Heydari-Malayeri & Lecavelier des Etangs 1994). Contursi et al. (2007) identified seven stars in the region in their near-infrared images, and suggested that their Star 2, Star 4, and Star 7 are YSOs (keeping with their nomenclature). They correspond to three definite YSOs identified by GC09. We detected three maser spots at different velocities toward Star 2. The feature which peaks at 272.5 km s−1 corresponds to the peak CO emission of the "blue" molecular cloud as identified in Heydari-Malayeri & Lecavelier des Etangs (1994). However, it is the "red" component which they believe to be more directly linked with the N4A region. Although it peaks at 278.5 km s−1, it would not be unusual for water masers to be spread by a mere 6 km s−1 from the systemic velocity (as is seen in Ellingsen et al. 2010).
Indebetouw et al. (2004) detected radio continuum emission at 4.8 GHz and 8.6 GHz toward Star 2. They suggested from the spectral index that the emission is attributed to thermal emission from an ultracompact H ii region caused by an O8 V star. This classification is consistent with a massive central object, but we assert this is still deeply embedded, based on the infrared analysis done by GC09 and Seale et al. (2009), as well as the presence of water masers, which normally indicate an early stage of formation (Ellingsen et al. 2010).
Water maser emission toward N4A had been recently discovered as reported by Imai et al. (2013). They used ATCA archival data from 2003, 5 yr earlier than the data presented here. They likewise detected water masers toward the GC09 massive YSO over a similar velocity range. The spectral characteristics are very different, but high variability is very common for water masers (Felli et al. 2007). Given only the two observations and time between them, it is impossible to know all the details of their variability. We can only assume that the maser emission here has been somewhat persistent at least over a 5 yr period (i.e., 2003–2008).
The maser spot peaking at 262.5 km s−1 is not associated with a YSO candidate or any radio continuum emission detected by Indebetouw et al. (2004), nor was it detected in the archival data by Imai et al. (2013; which may be simply due to variability). There is an infrared source detected in the Spitzer 3–8 μm IRAC data, but not at longer wavelengths, and there is also no associated 2MASS near-infrared point source. GC09 classified it as a diffuse source (J045205.39−665513.8) on the basis of it having no near- or far-infrared counterpart, and the fact that it is embedded in the aforementioned dust filament. The filamentary structure was identified by Heydari-Malayeri & Lecavelier des Etangs (1994). Even the spectral analysis by Seale et al. (2009) initially classified it as a highly embedded YSO, but they argued in favor of the GC09 interpretation, noting that such diffuse objects are "spectroscopically indistinguishable from YSOs" in that limited spectral range.
Given the corresponding infrared evidence, our detection of a water maser is strong evidence that this source is indeed a YSO. It is interesting to note that GC09 state that their diffuse sources may harbor low- or intermediate-mass YSOs. It is not uncommon for low-mass YSOs to exhibit water maser emission (Wilking et al. 1994, for example), but it would be the first detection of an extragalactic maser associated with a low-mass YSO (Imai et al. 2013). However, GC09 and Seale et al. (2009) both use the criterion that to first order, YSOs with an IRAC 8.0 μm magnitude less than eight ([8.0] < 8.0 mag) indicates they are high-mass. For J045205.39−665513.8, [8.0] = 7.74. In addition, Ellingsen et al. (2010) found that water maser emission in the LMC occurs preferentially toward the more luminous and redder GC09 definite YSOs. They identified criteria in color–magnitude space which identify regions where maser sources are most likely to be found, and the J045205.39−665513.8 colors satisfy these criteria. Although more study will be needed to determine its mass, we are certain it is a YSO.
4.2. N190
The emission nebula N190 is a poorly studied, inconspicuous H ii region near the NGC 1833 star cluster. Of the two masers detected here, one of them is associated with a GC09 massive YSO, but the other is about 4'' away. We investigated the 2MASS point-source catalog and found two sources, one which is the massive YSO (J05042500−7043444), but also another source (J05042559−7043434) that is separated from the far maser by 2''. Since the resolution of the 2MASS survey is 2'', we claim that this far maser is associated with it. 2MASS J05042559−7043434 has a very faint counterpart in the IRAC 4.5 μm data (Figure 3), but not in the other IRAC bands. This is reminiscent of extended green objects (EGOs) seen in the Milky Way, extended regions of emission often indicating shocked gas from massive YSO outflows (Cyganowski et al. 2013). Finding water masers toward these EGOs has been met with great success, where Cyganowski et al. (2013) detected water masers toward 68% of these structures. Naturally at the distance of the LMC, we do not have the resolution to determine if the source is a genuine EGO, but the associated 4.5 μm emission enhancement and water maser lead us to believe that 2MASS J05042559−7043434 is a previously undiscovered massive YSO.
The masers in N190 are new detections. They will be particularly beneficial to studies that propose to determine some of the dynamics of the LMC, such as the project discussed by Imai et al. (2013). With very long baseline interferometry and a sufficient number of masers, a statistical analysis of the maser proper motions can be used to determine the motion and galactic rotation of the LMC. With only small number of masers, each new star-forming region with masers will significantly improve such studies, each providing an additional three observables to fit to a dynamical model. More significantly, N190 is spatially separated from other known maser regions, useful in removing any degeneracy in the model fitting by sampling that area of the LMC. The nearest masers are separated by over 1
5, thus it is an essential probe of the galactic dynamics in that region. Including the new maser in N4A, these discoveries bring the total number of interstellar water masers in the LMC from 23 in 14 regions (Imai et al. 2013) to 26 in 15 regions.
4.3. N206
N206 is a high-mass star-forming region, where most of the star formation occurs along a molecular ridge at the edge of a superbubble formed by supernova shocks and massive stellar winds (Dunne et al. 2001; Kavanagh et al. 2012). Besides GC09, other groups have identified massive YSOs in N206, but with different results (i.e., GC09 identified 23, while Romita et al. 2010 and Carlson et al. 2012 identified 116 and 73, respectively). A search toward the five pointings in this region for water masers yielded a null detection down to around 20 mJy rms, thus we were unable to provide supporting evidence for these YSO candidates.
We do report detection of continuum emission at 22 GHz (Table 3) toward a GC09 massive YSO. Remarkably, neither Romita et al. (2010) nor Carlson et al. (2012) identified this source as a YSO candidate. Indebetouw et al. (2004) detected continuum emission at 4.8 GHz and at 8.6 GHz at this same location, but they determined a spectral index for a source slightly north of the massive YSO. Follow-up observations will be needed to determine a spectral index and thus the nature of the emission (i.e., whether it is thermal from a compact H ii region).
5. SUMMARY
A search for 22.2 GHz water masers from archival data toward three massive star-forming regions in the LMC resulted in the detection of four masers toward two regions. Three of these are new detections, bringing to date the total number of water masers around young stars in the LMC to 26 in 15 regions. New observations of N206 yielded no maser detections, but did reveal the presence of 22 GHz continuum emission, coincident with previously detected emission at lower frequencies (Indebetouw et al. 2004).
Two of the masers and continuum source are associated with massive YSOs identified by GC09 and confirmed by Seale et al. (2009), providing strong supporting evidence for their classification. One of the other masers was discovered toward a GC09 diffuse source in N4A, and one toward a 2MASS point source in N190. We assert that these masers are signaling the presence of YSOs. All the GC09 sources have 8.0 μm magnitudes less than 8.0, and have redder IRAC colors when compared to the YSO candidate sample as a whole. This supports the conclusions of Ellingsen et al. (2010), namely that massive YSOs with associated water maser emission are more luminous and have redder colors, indicating highly embedded objects. The other maser in N190 is not very bright in the IRAC bands, and therefore was not included in the GC09 survey. All in all, the methods used by GC09 appear to be successful in identifying massive YSOs. We recommend future surveys for water masers should target luminous, definite GC09 YSO candidates with infrared excess.
We thank Phil Edwards for allowing us Director's Time on the Australian Telescope Compact Array. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. We also acknowledge support from the NASA Rocky Mountain Space Grant and from the Brigham Young University Graduate Fellowship.
Facilities: Spitzer - Spitzer Space Telescope satellite, ATCA - Australia Telescope Compact Array