TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VIII. G12.89+0.49, G15.03–0.68 (M17), AND G27.36–0.16

Accurately determining the spiral structure of the Milky Way is a difficult task. In principle, kinematic distances can be used to construct the spiral structure of the Galaxy. However, recent work on parallax and proper motion measurements has shown that kinematic distances can be affected by large uncertainties. In some places of the Galaxy, characterized by "anomalous" motions (as, for example, the Perseus arm; Xu et al. 2006b), kinematic distances can be in error by a factor as large as two. Relying solely on kinematic distances, one cannot accurately determine the location of spiral arms. A more secure method of distance measurement is required to reconstruct the Galactic spiral structure and its three-dimensional motion (Reid et al. 2009b). Recently, CH₃OH 12 GHz masers have been used as astrometric targets to measure the trigonometric parallaxes and proper motions of massive star-forming regions (Reid et al. 2009a, hereafter called Paper I). As part of that large project, here, we present the results of our parallax measurement campaign toward the sources G12.89+0.49, G15.03−0.68, and G27.36−0.16.

A series of observations of the 12 GHz CH₃OH masers in the G12.89+0.49, G15.03−0.68, and G27.36−0.16 star-forming regions were carried out with the NRAO⁷ Very Long Baseline Array (VLBA). Paper I provides a description of the general observational setup and data calibration procedures, and here we note only details specific to the observations of the three sources presented in this paper.

G12.89+0.49 and G15.03−0.68 were observed at five epochs (VLBA program BR129A): 2007 October 18, 2008 April 17 and September 13, and 2009 March 23 and October 23. However, because of bad weather, the data quality of the first epoch was not good enough for parallax measurement, and the present results are based on the following four epochs only. G27.36−0.16 was observed at four epochs (VLBA program BR129C): 2007 October 27, 2008 April 24 and October 31, and 2009 April 16. The observing dates have been chosen to sample the peaks of the parallax signature in right ascension, as the amplitude of the parallax signature in declination is considerably smaller.

For each maser source, we used three different background sources, selected from the following calibrator surveys: the VLBI Exploration of Radio Astrometry (VERA; Honma et al. 2001) Galactic Plane Survey, a Very Large Array (VLA) survey of compact NVSS sources (Xu et al. 2006a), and the VCS2 and VCS3 catalogs (Fomalont et al. 2003; Petrov et al. 2005). Only background sources belonging to the VCS2 and VCS3 catalogs were detected. The non-detected calibrators were J1815−1717 and J1837−0628 from the VERA survey, and J1809−1804 and J1817−1614 from the VLA survey. Table 1 reports the peak positions and intensities of the maser and background sources, listing also the main observing parameters. We recorded four adjacent dual circularly polarized 4 MHz bands with the maser signal in the second band centered at the peak velocity of the 12 GHz maser emission, corresponding to an LSR velocity, V_LSR, of 40 km s⁻¹, 23 km s⁻¹, and 100 km s⁻¹ for sources G12.89+0.49, G15.03−0.68, and G27.36−0.16, respectively. The spectral resolution was 0.38 km s⁻¹.

Table 1. Positions and Brightnesses

Source	R.A. (J2000)	Decl. (J2000)		Brightness	VLSR	NW Beam
	(h  m  s)	(°   '   '')	(°)	(Jy beam−1)	(km s−1)	(mas, mas, deg)
G12.89+0.49	18 11 51.3955	−17 31 29.913		2.2	39.8	4.2×2.5 @ 2
J1825−1718	18 25 36.53228	−17 18 49.8484	3.3	0.14		5.7×3.8 @ −33
G15.03−0.68	18 20 24.8111	−16 11 35.316		2.9	23.4	3.6×1.8 @ −1
J1825−1718	18 25 36.53228	−17 18 49.8484	1.7	0.14		4.3×2.2 @ −11
G27.36−0.16	18 41 51.0570	−05 01 43.443		2.6	99.6	2.7×1.4 @ 16
J1834−0301	18 34 14.07456	−03 01 19.6274	2.8	0.07		2.8×1.7 @ 13
J1846−0651	18 46 06.30026	−06 51 27.7456	2.1	0.04		3.1×1.7 @ 20

Notes. is the angular separation between the maser and the calibrator. The maser absolute position, the peak brightness, the size, and position angle of the naturally weighted (NW) beam are listed for the epoch 2008 April 17 and April 24, for the source couple G12.89+0.49 and G15.03−0.68, and G27.36−0.16, respectively. The position angle of the beam is defined as east of north.

Download table as:  ASCIITypeset image

We used observations of the strong VLBA calibrators J1800+3848 and 3C345 (J1642+3948) to correct for instrumental delays and phase offsets among different frequency bands. The spectral channel with the strongest maser emission was used as the phase reference (see Table 1). The maser reference features were detected at all epochs and were relatively stable in flux, with the exception of the source G12.89+0.49, whose correlated flux density on the shortest VLBA baselines increased monotonically by a factor of ≈2, while Goedhart et al. (2009) found a periodical variation with a period of ~30 days and flux variations of ~50%. After applying the maser calibration, we integrated the data of the background continuum sources over all four dual-polarized bands and imaged the sources using the AIPS task IMAGR. For all maser targets, the maser-referenced calibrator image has a dynamical range of about 10.

The naturally weighted "dirty" beam was determined by the availability of maser phase-reference data and, as expected for low-declination sources, was strongly elongated close to the N–S direction. For each of the three maser targets, Table 1 reports the parameters of the "dirty" beam as derived by the AIPS task IMAGR by fitting an elliptical Gaussian brightness distribution to the central portion of the "dirty" beam map. The images of the maser and corresponding calibrators have been restored using a round "clean" beam, with an FWHM size intermediate between the minor and major FWHM sizes of the "dirty" beam. To make images from different epochs more readily comparable, maps of a given source were restored using the same beam.

After obtaining maps of both maser and background sources for all epochs, we derived the parameters of the emission by fitting elliptical Gaussian brightness distributions. Table 1 lists the position and peak intensity of the maser and corresponding calibrator(s), as well as the angular separation on the sky between the two sources. Absolute maser positions are derived from the positions of the ICRF sources J1825−1718 and J1834−0301, which are accurate within ≈1 mas.

2.1. Methanol Masers

Following the analysis described in Paper I, the measured positions of the masers were modeled as a linear combination of the elliptical parallax and linear proper motion signatures. Because systematic errors (owing to small uncompensated atmospheric delays and, in some cases, varying maser and calibrator source structures) typically dominate over thermal noise when measuring relative source positions, we added "error floors" in quadrature to the formal position uncertainties. We used different error floors for the right ascension and declination data and adjusted them to yield post-fit residuals with χ² per degree of freedom near unity for both coordinates.

3.1. G12.89+0.49

Figure 1 presents the images of the G12.89+0.49 reference maser channel at V_LSR = 39.8 km s⁻¹ and the background continuum source J1825−1718 (phase referenced to the reference maser channel) at the second epoch (2008 April 17). Both the maser and the background source emission are dominated by a single compact component, which serves as an astrometric target. Figure 2 reports the positions of the emission in the reference maser channel (relative to the background source J1825−1718) as a function of time and the parallax fit. The large declination errors of the parallax fit are probably caused by residual phase errors due to the combination of the large maser–calibrator separation (33) and the low maser declination.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Fitting for the parallax and proper motion simultaneously, we obtain π = 0.428 ± 0.022 mas. The proper motions in the eastward and northward directions are 0.16 ± 0.03 and −1.90 ± 1.59 mas yr⁻¹, respectively (see Table 2). This parallax corresponds to a distance of 2.34^+0.13_−0.11 kpc. Based on this distance, G12.89+0.49 is most likely in the Carina–Sagittarius arm of the Milky Way, and not in the Crux–Scutum arm.

Table 2. Parallax and Proper Motion Fit

Maser	VLSR	Background	Parallax	μx	μy
Name	(km s−1)	Source	(mas)	(mas yr−1)	(mas yr−1)
G12.89+0.49	39.8	J1825−1718	0.428 ± 0.022	0.16 ± 0.03	−1.90 ± 1.59
G15.03−0.68	23.4	J1825−1718	0.505 ± 0.033	0.68 ± 0.05	−1.42 ± 0.09
G27.36−0.16	92.2	J1834−0301	0.125 ± 0.042	−1.81 ± 0.08	−4.11 ± 0.26
	92.2	J1846−0651	0.198 ± 0.133	−1.90 ± 0.27	−4.75 ± 0.15

Download table as:  ASCIITypeset image

3.1.1. Maser Environment

G12.89+0.49, better known as IRAS 18089−1732, has long been known as a prominent maser source, showing strong emission from all the widespread interstellar maser molecules (OH, H₂O, and CH₃OH). It was included in the sample of high-mass protostellar objects defined by Sridharan et al. (2002) and further studied by Beuther et al. (2002a and references therein). Except for a single spot (the component F) detached by 14, all of the 6.7 GHz CH₃OH maser spots detected by Walsh et al. (1998) coincide with our VLBA maser position within the errors (~1'') of the Australia Telescope Compact Array (ATCA). Our VLBA maser emission is most likely corresponding to the component A of Walsh et al. (1998), which shows a similar V_LSR = 39.2 km s⁻¹. H₂O maser emission has been found with the VLA ≈ 12 to the NNE (Beuther et al. 2002b). Interestingly, the H₂O maser position coincides with weak centimeter-wavelength radio continuum emission and a (sub)millimeter dust emission peak, while the CH₃OH maser is offset from these peaks (Beuther et al. 2004). The masers and continuum emission are associated with a rich molecular hot core, which has been interpreted as a rotating disk (Beuther et al. 2005).

3.2. G15.03−0.68

Figure 3 presents the images of the G15.03−0.68 reference maser channel (V_LSR = 23.4 km s⁻¹) and the background continuum source J1825−1718 (phase referenced to the reference maser channel), at the second epoch (2008 April 17). Figure 4 reports the positions of emission in the reference maser channel (relative to the background source J1825−1718) as a function of time and the parallax fit.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Fitting for the parallax and proper motion simultaneously, we obtain π = 0.505 ± 0.033 mas, corresponding to a distance of 1.98^+0.14_−0.12 kpc. At this distance, G15.03−0.68 is likely in the Carina–Sagittarius arm. The proper motions in the eastward and northward directions are 0.68 ±  0.05 and −1.42 ±  0.09 mas yr⁻¹, respectively (see Table 2).

3.2.1. Maser Environment

The maser position coincides with the "unusual radio point source in M17" found by Felli et al. (1980), one of the first ultracompact H ii regions ever identified as such. The position of the 6.7 GHz maser emission, as recently determined by Caswell (2009) using ATCA, is consistent within the ATCA measurement error (≈1'') with what we determine for the 12.2 GHz line.

3.3. G27.36−0.16

Figure 5 presents the images of the G27.36−0.16 reference maser channel (V_LSR = 99.6 km s⁻¹) and the two background continuum sources J1834−0301 and J1846−0651 (phase referenced to the reference maser channel) at the second epoch (2008 April 24). Figure 6 reports the positions of the reference maser channel (relative to the background sources J1834−0301) as a function of time and the parallax fit.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

For this source, the derived value of parallax has a large fractional uncertainty. Using only data from the J1834−0301 calibrator, we obtain π = 0.125 ± 0.042 mas, while using the J1846−0651 data, we find π = 0.198 ± 0.133 mas. Since the result from the J1834−0301 calibrator appears significantly more precise than that obtained from J1846−0651, we take the former as the best parallax measurement for G27.36−0.16, corresponding to a distance of 8.0^+4.0_−2.0 kpc. The proper motions in the eastward and northward directions are −1.81 ± 0.08 and −4.11 ± 0.26 mas yr⁻¹, respectively (see Table 2).

3.3.1. Maser Environment

The 12.2 GHz CH₃OH maser in the G27.36−0.16 region is offset by about 2'' from a compact <2'' radio source found by Becker et al. (1994), which also has an associated IRAS source (18391−0504). Much stronger emission than that in this line has been found in the 6.7 GHz CH₃OH line (Szymczak et al. 2002), which has associated maser emission from OH and H₂O (Szymczak & Gérard 2004; Szymczak et al. 2005). This source appears to have been little studied in thermal molecular line emission.

Combining the distances, LSR velocities, and proper motions of the masers yields their locations in the Galaxy and their full space motions. Since internal motions of 12 GHz methanol masers are fairly small, typically ~3 km s⁻¹ (Moscadelli et al. 2002), the maser motions should be close to that of their associated young stars. Given a model for the scale and rotation of the Milky Way, we can subtract the effects of Galactic rotation and the peculiar motion of the Sun from the space motions of the maser sources and estimate the peculiar motions of the associated young stars. Following the discussion by Reid et al. (2009b), the motion of an individual massive star (associated with the masers) can then be taken as representative of that of the whole star-forming region to which the massive star belongs, allowing for a velocity dispersion of individual stars of ≈7 km s⁻¹ per each velocity coordinate. Peculiar motions are given in a Galactocentric reference frame, where U, V, and W are the velocity components toward the Galactic center, in the direction of Galactic rotation, and toward the North Galactic Pole, respectively, at the location of a given source in the Galaxy. We adopt the IAU values for the distance to the Galactic center (R₀ = 8.5 kpc) and the rotation speed of the Galaxy at this distance (Θ₀ = 220 km s⁻¹), and assume a flat rotation curve. We use the solar motion value (U = 11.1^+0.69_−0.75, V = 12.24^+0.47_−0.47, and W = 7.25^+0.37_−0.36 km s⁻¹) by Schönrich et al. (2010), who have recently revised the Hipparcos satellite result. Table 3 reports the peculiar motions derived for our maser targets. For comparison, the results obtained with a different model of Galactic rotation (R₀ = 8.3 ± 0.23 kpc, Θ₀ = 239 ± 7 km s⁻¹), based on both the weighted average of four recent direct measurements of the distance to the Galactic center and the proper motion of Sgr A* (Brunthaler et al. 2011), are also reported. Looking at Table 3, one can see that, except for G27.36−0.16, whose space motion may be strongly affected by the large parallax uncertainty, the derived peculiar motions are fairly independent of the adopted Galactic rotation model. For all sources, the motion directed toward the north Galactic pole is only of a few km s⁻¹. This is consistent with the expectation for the motion of massive star-forming regions, which should be mainly in the Galactic plane.

We have measured the parallax and proper motion of 12 GHz methanol masers in three star-forming regions. For two sources, the derived distances are accurate by better than 10%: 2.34^+0.13_−0.11 kpc for G12.89+0.49 and 1.98^+0.14_−0.12 kpc for G15.03−0.68. For the source G27.36−0.16, the derived distance is affected by a large uncertainty: 8.0^+4.0_−2.0.

Our precise absolute positions place the methanol masers near the center of active regions of high-mass star formation, as traced by molecular hot cores, ultracompact H ii regions and/or dust condensations, or a combination of these.

This work was supported by the Chinese NSF through grants NSF 11073054, NSF 10733030, NSF 10703010 and NSF 10621303, and NBRPC (973 Program) under grant 2007CB815403.

Facility: VLBA - Very Long Baseline Array