Organic chemistry around young high-mass stars
Allen, Veronica Amber
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Allen, V. A. (2018). Organic chemistry around young high-mass stars: Observational and theoretical. University of Groningen.
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Chapter 4
Mechanical properties of the
molecular outflows from the
high-mass disk candidates
G35.20-0.74 and G35.03+0.35
V. Allen, F. F. S. van der Tak, Á. Sánchez-Monge, R. Cesaroni, M. T. Beltrán (In preparation)Abstract
Context: Disks and outflows are necessary to solve the angular momen-tum problem, especially in high-mass star formation where the radiation pressure is high enough to disrupt accretion before enough mass can be gathered. Several potential Keplerian disks have been detected in high-mass star-forming regions.
Aims: We have previously studied the disk candidates associated with G35.20-0.74N (G35.20) and G35.03+0.35 (G35.03) in particular and now aim to image their associated outflows to test the presence of an accretion disk and determine the number of sources within.
Methods: We observed the two star-forming regions with the NOEMA interferometer and complementary observations with the IRAM 30 m telescope. To determine the properties of the outflows we focused on
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outflow-tracing species HCO+ (1-0) and SiO (2-1) with a spectral reso-lution of 0.6 km s−1 and an angular resolution of ∼2.500 ( pc).
Results: We detect bipolar outflows around G35.20 and G35.03 in HCO+emission while SiO emission is weaker and the wings are not well resolved. There appear to be two outflows associated with G35.20 B, one with a north-south orientation originating at the main continuum peak of hot core G35.20 B and the other with a northeast-southwest orientation originating at the another continuum peak in G35.20 B which is approximately perpendicular to the observed rotation. The outflow emission at G35.03 is less clear due to a complex velocity field, but appears to have an approximately north-south orientation. We derive the mechanical properties of the outflows which agree well with published trends.
Conclusions: The properties of the outflows including mechanical luminosity, kinetic energy output, and mass-loss rate are well within ex-pected ranges based on the work of Wu et al. (2004). The luminosity of the outflows account for ∼ 1% of the bolometric luminosity in G35.20 and less than 0.1% in G35.03 with outflow masses of 2-5 M per lobe. The outflow associated with G35.03 does not have a clear bipolar struc-ture originating from the hot core, but does show emission perpendicular to the proposed disk. Our observations thus support the hypothesis that G35.20 and G35.03 have developed disk-outflow systems similar to low-mass star formation.
4.1 Introduction
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4.1
Introduction
Several models have been proposed to explain the earliest processes in high-mass star formation – while McKee & Tan (2003) suggest that the process is similar to that of low-mass stars, Bonnell & Smith (2011) pro-pose that stars in a cluster compete for the surrounding matter while funneling more material to the most massive core, and Keto (2007) sug-gest that mass flows onto the protostar in gravitationally trapped hy-percompact HII regions (for a review see Tan et al. 2014). All of these models predict the existence of disks to allow matter to accrete onto the protostar despite high radiation pressure (Krumholz et al. 2009) and bipolar outflows are expected to accompany them allowing radia-tion to escape so that accreradia-tion can proceed. Recently several candidate disks have been identified around B-type protostars (Cesaroni et al. 2006; Wang et al. 2012; Sánchez-Monge et al. 2013a; Beltrán et al. 2014) and O-type (proto)stars (Kraus et al. 2010; Johnston et al. 2015; Cesaroni et al. 2017). See Beltrán & de Wit (2016) for a review of disks around high-mass (proto)stars. Signposts of outflow activity include molecular emission lines from H2O, CO, and SiO (among others) with broad veloc-ity wings showing the high speeds attained in the outflow – at least 10 km s−1 and as high as 100s of km s−1 (van der Tak et al. 2013; San José-García et al. 2016). SiO emission arises in shocks from dust vaporization (Guillet et al. 2009), dust collisions (Caselli et al. 1997), or sublimation from ices (Gusdorf et al. 2008).
This work focuses on two molecular clouds that we previously studied in the process of fragmenting into several cores, some of which clearly show high-mass star formation. In particular, Keplerian disk candi-dates have been found in G35.20-0.74N (Sánchez-Monge et al. 2013a, 2014), hereafter G35.20, and G35.03+0.35 (Beltrán et al. 2014), here-after G35.03. The star-forming region G35.20 is located at a distance of 2.2 kpc and has a bolometric luminosity of 3.0×104 L (Sánchez-Monge et al. 2014). The 870 µm continuum shows six condensations of mass including two hot cores – G35.20 A and G35.20 B. These two sources appear to have a coherent rotating structure and show the chemical com-plexity of a hot core with over 30 different molecular species detected. The star-forming region G35.03 is located at a distance of 2.34 kpc (Wu et al. 2014) and has a bolometric luminosity of 1.2×104 L (Beltrán
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et al. 2014). The 870 µm continuum shows six cores including one hot core – G35.03 A. As with G35.20, the hot core in G35.03 shows a coher-ent rotating structure in several differcoher-ent molecular tracers and a similar chemical complexity. The high-mass hot cores in G35.20 show velocity gradients, of which one follows a Keplerian disk model while the other is too compact to be resolved and characterized in detail, but is indicative of disk rotation. In G35.03, the high-mass hot core is consistent with Keplerian rotation as demonstrated by the butterfly-shaped pattern in the corresponding position-velocity plot.
Figure 4.1.1: Integrated intensity contours of HCO+ for the blue outflow lobe (15
-25 km s−1) and dashed contours for the red outflow lobe (36 - 50 km s−1) in G35.20 overlaid on the enhanced 4.5 µm Spitzer map (Sánchez-Monge et al. 2014) showing shocked H2with the black contours showing the ALMA 870 µm (350 GHz) continuum
for reference. The hot cores G35.20 A and G35.20 B are indicated and the other mass condensations lie along the apparent filament shown by the ALMA continuum.
Large surveys of molecular outflows (Cabrit & Bertout 1992; Shep-herd & Churchwell 1996; Beuther et al. 2002; Wu et al. 2004;
López-4.2 Observations
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. . . .
Sepulcre et al. 2009; Sánchez-Monge et al. 2013b) have revealed trends between the derived outflow properties (outflow mass vs bolometric lu-minosity, outflow mass vs dynamic time, mechanical luminosity vs bolo-metric luminosity, and others – see Appendix 4.A). In this work, we aim to detect and characterize outflows associated with our disk candidate sources to see how they compare to other known sources. Detecting out-flows associated with Keplerian disks takes the next step in confirming the presence of a disk and providing evidence that high-mass stars form in a disk-outflow system. The goal of our previous the ALMA obser-vations (Sánchez-Monge et al. 2013a, 2014; Beltrán et al. 2014; Allen et al. 2017) was to image disks around B-type protostars and therefore were only sensitive to compact structures (< 200) corresponding to the maximum size we would expect for the disks. This instrumental config-uration allowed us to detect and study the disks in the two regions, but the molecular outflows (with angular scales of several arcsec) were mostly resolved out. Previous sub-millimeter line images of G35.20 made with the James Clark Maxwell telescope (JCMT) and the Berkeley-Illinois-Maryland Association (BIMA) and G35.03 with the Atacama Submil-limeter Telescope Experiment (ASTE) (Gibb et al. 2003; Paron et al. 2012) show evidence for outflows, but do not resolve them spatially. The current study focuses on the larger scale at higher angular resolution than these previous images to resolve the outflows and derive their mechanical properties.
4.2
Observations
Observations were taken on 8 December 2015 with seven 15 m anten-nae at NOEMA1 in Configuration C (baselines between 20 and 192 m) attaining a resolution of ∼ 2.500 in order to separate multiple outflows and bridge the gap between our ALMA observations with a beam size of ∼ 0.500 (Sánchez-Monge et al. 2014; Beltrán et al. 2014; Allen et al. 2017) and previous outflow studies at about 1000done with smaller interferom-eters (BIMA and ASTE) and with single dish telescopes (JCMT) (Gibb et al. 2003; Paron et al. 2012). Additional observations were taken in April 2016 using the IRAM 30 m telescope to fill in the short spacings and gain sensitivity to extended emission on large scales. The spectral
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Figure 4.1.2: Integrated intensity contours of HCO+ for the east-west component (blue contours) and northeast-southwest component (red contours) in G35.03 overlaid on the 4.5 µm Spitzer map showing shocked H2 with the black contours showing the
ALMA 870 µm (350 GHz) continuum for reference. The blue contours show the integrated intensity between 44 and 46 km s−1 with contours from 5σ (66 mJy) to 85 mJy. The red contours show the integrated intensity between 52 and 54 km s−1 with contours from 5σ (0.9 Jy) to 1.81 Jy. The hot core G35.03 A is indicated and the other mass condensations are seen within the ALMA continuum.
resolution is 0.54 km s−1with a channel spacing of 156 kHz (512 channels in dual sidebands for a bandwidth of each spectral window of 80 MHz) for NOEMA. For the 30 m telescope observations, we used the EMIR E090 band and FTS200 as the backend to achieve a spectral resolution of around 0.6 km s−1 in an on-the-fly map of 9 arcmin2. The data were reduced in Grenoble, France, using the GILDAS software, combining the reduced and regridded 30 m data with the NOEMA observations for the 71.875 MHz spectral windows containing the SiO (2-1) transition at 86850 MHz and the HCO+ (1-0) transition at 89190 MHz.
4.3 Results
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4.3
Results
4.3.1 HCO+ maps
Figure 4.1.1 presents G35.20 (systemic vLSR=30 km s−1) with the con-tours of the HCO+ blueshifted emission wings (15-25 km s−1) and the redshifted emission wings (36-50 km s−1) overlaid on a map of H2 emis-sion at 4.5 µm. The H2 emission shows where shocks from the sources within are interacting with the surrounding molecular cloud. In this source, the HCO+ emission coincides well with the H2 emission. From this plot we can see that there appear to be two outflow components: one nearly north-south and one northeast-southwest.
Figure 4.1.2 shows the contours of HCO+emission from G35.03 over-laid on the 4.5 µm Spitzer map of H2 emission. The systemic velocity of the hot core G35.03 A is 45 km s−1, but the outflow tracer emission peaks around 54 km s−1. There are two distinct components shown: one in an east-west direction (vLSR 49-52 km s−1), which seems to be associated with the hot core, and one in a northeast-southwest direction (53-56 km s−1) which aligns with the filament seen in the 870 µm continuum.
4.3.2 SiO maps
Figure 4.3.1 shows integrated intensity maps of SiO emission in G35.20 for four different velocity ranges. Both the blue- and red-shifted emission appears mainly to the south of the ALMA continuum source with the peaks tracing the regions without H2 emission in the Spitzer image. Fig-ure 4.3.2 shows similar maps for G35.03, where there is consistently SiO emission to the southeast of the ALMA continuum tracing the dust lanes (white regions) in the Spitzer image. There is another component that appears to trace the ALMA continuum with an approximately north-south orientation in a similar manner to the HCO+emission between 52 and 54 km s−1 shown in Figure 4.1.2.
4.3.3 H13CO+ emission
H13CO+ is detected toward both sources, but, as it is much weaker than the HCO+ emission, does not trace the high velocity gas. The maximum velocity in the H13CO+ emission is 10 km s−1 from systemic in G35.20 and 4 km s−1 from systemic for G35.03. In Figure 4.3.3 we
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Figure 4.3.1: Integrated intensity maps for SiO toward G35.20 overlaid on 4.5µm H2
emission with black contours showing 870 µm ALMA continuum. (Top right) Blue contours (181 - 351 mJy/beam km s−1) with the velocity range 20-27 km s−1, (top left) Cyan contours (97 - 293 mJy/beam km s−1) with the velocity range 27-30 km s−1, (bottom right) Red contours (164 - 381 mJy/beam km s−1) velocity range 34-36 km s−1, and (bottom left) Magenta contours (141 - 321 mJy/beam km s−1) velocity range 36-42 km s−1.
4.3 Results
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. . . .
Figure 4.3.2: Integrated intensity maps for SiO toward G35.03 overlaid on 4.5µm H2
emission with black contours showing 870 µm ALMA continuum. (Right to Left) Cyan contours (38 - 193 mJy/beam km s−1) with the velocity range 44-46 km s−1, Green contours (86 - 398 mJy/beam km s−1) with the velocity range 46-50 km s−1, Red contours (134 - 615 mJy/beam km s−1) velocity range 50-54 km s−1, and Magenta contours (84 - 415 mJy/beam km s−1) velocity range 56-60 km s−1.
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show the spatial extent of the H13CO+ emission which seems to mainly trace the dense gas of the filaments (as illustrated with the ALMA band 7 continuum).
Figure 4.3.3: H13CO+contours (cyan) and ALMA 870 µm continuum contours
(ma-genta) overlaid on 4.5 µm Spitzer image (greyscale) for G35.20 (left) and G35.03 (right).
4.4
Outflow properties
4.4.1 Methodology
We calculate the mechanical properties of the outflows associated with G35.20 and G35.03 based on the integrated intensity maps of the red and blue wings of the HCO+ lines as in Sánchez-Monge et al. (2013b) and López-Sepulcre et al. (2009). The measured properties for each outflow lobe are: the difference between the maximum velocity and systemic velocity (vmax), the length of the major axis (at the 5 σ contour), the area of the outflow (as an ellipse), and the integrated intensity of the emission between vmax and 2σ from the line peak (R Tmbdv).
The HCO+column density was calculated from the integrated HCO+ (1-0) line emission assuming optically thin emission as in Goldsmith & Langer (1999) equations (10) and (19) using excitation temperatures of 10, 20, and 30 K (based on the typical dust temperatures at these scales). The calculated HCO+ column densities varied by a factor of 3-4
4.4 Outflow properties
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. . . .
with the different temperatures so we used the average for calculating the hydrogen column density (NH2) assuming an HCO+ abundance of 10−9, as abundances of 1-9×10−9 have been used in the past (Irvine et al. 1987; Tafalla et al. 2010; Busquet et al. 2011; Tafalla & Hacar 2013; Sánchez-Monge et al. 2013b)).
The other calculated properties are: outflow mass (calculated using the area of the outflow lobe and the mass of the hydrogen from the calculated H2 column density), the dynamical or kinetic time – tdyn (the length of the outflow lobe divided by vmax), kinetic energy (outflow mass times 0.5 v2max), momentum (outflow mass times vmax), momentum rate – which is also called the force(momentum/tdyn), mass-loss rate (outflow mass/tdyn), and mechanical luminosity (kinetic energy/tdyn). For G35.20 the integrated intensity maps for -10-25 km s−1 and 35-80 km s−1 were used to determine the outflow properties and for G35.03 the integrated intensity maps for 30-53 km s−1 and 55-70 km s−1 were used.
4.4.2 Results
Table 4.4.1 summarizes the calculated and measured properties of the outflows from G35.20 and G35.03 as derived from HCO+ emission. As the more luminous source, G35.20 generally has higher calculated prop-erties. The N-S component is the more energetic and luminous of the two outflow components in G35.20 with a total outflow luminosity of ∼270 L compared to ∼70 L from the NE-SW component and <10 L for the outflow emission toward G35.03. The masses of the outflows are similar at ∼10, 8.5, and 7 M for G35.20 N-S, G35.20 NE-SW, and G35.03 respectively. The dynamic timescales for G35.20 are shorter than G35.03 at 5-11 kyr compared to 13-16 kyr. Mass-loss rates are of the same order for all observed sources in the range 2-12 ×10−4 M yr−1.
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T able 4.4.1: Measured and deriv ed outflo w prop erties for the tw o outflo ws asso ciated with G35.20 and the outflo w asso ciated with G35 .03 with errors in pare n theses. G35.20 N-S G35.20 NE-SW G35.03 Red Blue Red Blue Red Blue Outflo w mass (M ) 5.9 (0.5) 3.7 (0.5) 5.3 (0.4) 3.2 (0.5) 4.3 (0.3) 2.7 (0.3) Length (p c) 0.210 (0.008) 0.239 (0.008) 0.345 (0.008) 0.328 (0.008) 0.232 (0.008 ) 0.213 (0.008) v max (km s− 1 ) 42.7 (0.6) 4 1. 6 (0.6) 31.6 (0.6) 32.0 (0. 6 ) 14.1 (0.6) 15.8 (0.6) t dy n (yr × 10 3 ) 4.8 (0.2) 5.6 (0.2) 10.7 (0.3) 10.0 (0.3) 16.2 (0.9) 13.2 (0.7) Momen tu m (M km s− 1 ) 251 (20) 156 (20) 167 (14) 101 (15) 60 (5) 40 (5) Kinetic Energy (erg × 10 46 ) 10.6 (0.9) 6.4 (0.8) 5.3 (0.4) 3.2 (0.5) 0.84 (0.06) 0.67 (0.08) Momen tum rate (M km s− 1 yr − 1 ) 0.052 (0.005) 0.028 (0.004) 0.016 (0.001) 0.010 (0.002) 0.0037 (0.0004) 0.0030 (0.0004) Mec hanical Luminosit y (L ) 169 (15) 102 (13) 42 (4) 26 (4) 4.8 (0 .4) 3.8 (0.5) Mass-loss rate (M yr − 1 × 10 − 4 ) 12 (1) 6.6 (0.9) 5.0 (0.4) 3.2 (0.5) 2.6 (0.2) 2.0 (0.3)4.5 Discussion
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. . . .
Figure 4.4.1: Illustration of projected outflow orientation in G35.20 with dotted lines indicating the N-S component and dot-dashed lines indicating the NE-SW component. Integrated intensity contours of HCO+for the blue (-10 - 25 km s−1
) and red (35 - 80 km s−1) outflow lobes in G35.20. The blue contours start at 5σ (1.83 Jy/beam km s−1) and peak at 5.04 Jy/beam km s−1and the red contours start at 1.84 Jy/beam km s−1and peak at 5.23 Jy/beam km s−1. (inset) Velocity gradient (first moment) map of CH3CN(19-18) K=2 emission in G35.20 (colorscale from 28-33 km s−1) with 3.6 cm
free-free continuum emission (red contours) and outflow orientation lines (dotted and dot-dashed as in main figure). (first moment map from Sánchez-Monge et al. 2013a)
4.5
Discussion
4.5.1 The nature of the G35.20 outflows
We propose that there are two separate outflows associated with G35.20 B: one in the north-south direction and one in the northeast-southwest direction (illustrated in Figure 4.4.1). In Figure 4.4.1, we illustrate the approximate paths of the N-S and NE-SW outflow components. Both seem to originate at G35.20 B near 3.6 cm continuum emission with
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the N-S component passing through the south-east part and the NE-SW component passing through the center, perpendicular to the rotation direction. This agrees with the results in Sánchez-Monge et al. (2014) and Allen et al. (2017) that describe G35.20 B as a multiple system separated by ∼1000 AU with a circum-multiple disk. While this appears to follow from the HCO+(1-0) morphology (Figure 4.1.1) we cannot rule out the possibility that all of the HCO+ emission observed toward this source is associated with the proposed precessing jet (Sánchez-Monge et al. 2014) associated with G35.20 B.
The SiO emission shown in Figure 4.3.1 appears to trace the dark lane from the Spitzer image so could be associated with a different shock. This emission is much weaker than the HCO+ emission and does not sample the wings well as SiO is only detected at velocities <25 km s−1 from systemic, while HCO+is seen out to 40 km s−1from systemic. Similarly, the H13CO+emission only appears to trace the dense gas along the main filament, so we use HCO+ as a more effective tracer.
It was proposed in Sánchez-Monge et al. (2014) that the SiO (8-7) emission associated with G35.20 A traces an outflow in the northeast-southwest direction with a 3.6 cm source toward the northeast-southwest. We know from ALMA band 6 observations of G35.20 that the 3.6 cm continuum source coincides with H30α emission (private communication Patricio Sanhueza) and is therefore likely to be a separate source, but there does appear to be a component of the blue-shifted HCO+ emission that co-incides with the SiO outflow in Sánchez-Monge et al. (2014), so it may still trace the outflow from G35.20 A.
4.5.2 G35.03
It is unclear whether the HCO+ and SiO emission toward G35.03 are tracing outflows or other dense gas/shocks. The brightest component of the SiO emission in G35.03 is always to the southeast of the ALMA 870 µm continuum (Figure 4.3.2), though the component which appears to follow the continuum has an approximately north-south orientation. The range of velocities in both the SiO and HCO+ emission (40-70 km s−1peaking at 54 km s−1) is not well associated with the average velocity of the hot core in G35.03 (∼ 45 km s−1). It is likely that since we are probing the large scale gas (>200or ∼ 500 AU), the NOEMA observations are tracing the dense gas unassociated with the hot core.
4.5 Discussion
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. . . .
4.5.3 Outflow Properties
The relationships between mechanical outflow properties can be used to determine whether an outflow is driven by momentum (with fast or slow winds) or energy (Cabrit & Bertout 1992). Large studies searching for trends between these properties have shown that the relationships are the same across several orders of magnitude in mass demonstrating that outflows are similar in low- and high-mass star formation (Wu et al. 2004).
G35.20 was studied by López-Sepulcre et al. (2009) as part of their survey of outflows in high-mass young stellar objects using CO isotopo-logue lines. Our total outflow mass is smaller (16-20 M vs 110-230 M ), but they sample a much larger area with CO emission (with outflow sizes of 0.8 and 1.4 pc compared to our 0.2-0.3 pc). Our momentum values are 2-3 times smaller and mechanical luminosity is 1-2 orders of magnitude higher. Kinetic energy values and mass-loss rate are of a similar order. The dynamic timescale for the NE-SW outflow is of a similar order to that reported in López-Sepulcre et al. (2009), but the dynamic timescale for the N-S component is about half as long.
The outflows associated with G35.03 were previously studied by Paron et al. (2012) using12CO,13CO, and HCO+emission. Our maximum ve-locities match their 12CO maximum velocities and our derived HCO+ abundance is very similar to theirs (1×1012vs. 9×1012. They used their 12CO emission to determine outflow parameters, and we have very sim-ilar values for the red lobe (∼ 5 M , momentum of 60-70 M km s−1, kinetic energy 425-510 M (km s−1)2) but their blue lobe is roughly an order of magnitude higher on these mechanical properties.
Relationships between the mechanical properties of outflows were studied in detail by Wu et al. (2004) who found correlations between dif-ferent properties (eg. mechanical luminosity vs. bolometric luminosity) though these correlations can have a scatter of up to 2 orders of magni-tude. In Appendix 4.A, we compare our results to the relationships from Wu et al. 2004. Our calculated outflow properties fall well within the scatter of these plots and for force vs. bolometric luminosity, our results fall very near the best-fit line.
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4.6
Conclusions
We detect two outflows associated with G35.20 in HCO+emission which appear to originate from the proposed circumbinary Keplerian disk in the hot core region G35.20 B. The northeast-southwest component of this outflow is approximately perpendicular to the rotation observed in this source, which strengthens the case for the disk. The outflow tracer emission around G35.03 is complicated, but seems to align with the outflow orientation described in Beltrán et al. (2014). As most of the detected emission in G35.03 is red-shifted with respect to the hot core, it is unclear whether it is associated with an outflow driven by a source at G35.03 A. Further observations of these sources with higher sensitivity or longer exposure time may reveal the high velocity wings of the SiO (1-0) emission, but higher energy transitions may be better for tracing the emission from energetic outflows. The complexity of G35.03 can be further studied using CO isotopologues at high angular resolution which may provide clarity in this region.
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Appendices
Appendix 4.A
Comparing Outflow Properties with
Wu et al. (2004)
Figure 4.A.1: The mechanical luminosity (Lm) vs. bolometric luminosity (Lbol) for
G35.20 (red circles) and G35.03 (purple circles) overlaid on the results of Wu et al. (2004).
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Figure 4.A.2: The outflow mass (M) vs. Lbol for G35.20 (red circles) and G35.03
4.A Comparing Outflow Properties with Wu et al. (2004)
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. . . .
Figure 4.A.3: The outflow mass (M) vs. dynamic time (t) for G35.20 (red circles) and G35.03 (purple circles) overlaid on the results of Wu et al. (2004).
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Figure 4.A.4: The momentum rate (force – F) vs. bolometric luminosity (Lbol) for
G35.20 (red circles) and G35.03 (purple circles) overlaid on the results of Wu et al. (2004).
4.B Channel Maps
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Appendix 4.B
Channel Maps
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4.B Channel Maps
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Figure 4.B.3: Channel maps for HCO+ toward G35.03.
