High resolution AMI Large Array imaging of spinning dust sources: spatially correlated 8 micron emission and evidence of a stellar wind in L675
Anna M. M. Scaife, David A. Green, Guy G. Pooley, Matthew L. Davies, Thomas M. O. Franzen, Keith J. B. Grainge, Michael P. Hobson, Natasha Hurley-Walker, Anthony N. Lasenby, Malak Olamaie, John S. Richer, Carmen Rodriguez-Gonzalvez, Richard D. E. Saunders, Paul F. Scott, Timothy W. Shimwell, David J. Titterington, Elizabeth M. Waldram, Jonathan T. L. Zwart
aa r X i v : . [ a s t r o - ph . GA ] J a n Mon. Not. R. Astron. Soc. , 1–5 (2009) Printed 31 October 2018 (MN L A TEX style file v2.2)
High resolution AMI Large Array imaging of spinning dust sources:spatially correlated 8 m m emission and evidence of a stellar wind inL675 ⋆ AMI Consortium: Anna M. M. Scaife , †, David A. Green , Guy G. Pooley ,Matthew L. Davies , Thomas M. O. Franzen , Keith J. B. Grainge , ,Michael P. Hobson , Natasha Hurley-Walker , Anthony N. Lasenby , ,Malak Olamaie , John S. Richer , , Carmen Rodr´ıguez-Gonz´alvez ,Richard D. E. Saunders , , Paul F. Scott , Timothy W. Shimwell ,David J. Titterington , Elizabeth M. Waldram & Jonathan T. L. Zwart Astrophysics Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland Kavli Institute for Cosmology Cambridge, Madingley Road, Cambridge, CB3 0HA Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York 10027, USA
Accepted —; received —; in original form 31 October 2018
ABSTRACT
We present 25 ′′ resolution radio images of five Lynds Dark Nebulae (L675, L944, L1103,L1111 and L1246) at 16 GHz made with the Arcminute Microkelvin Imager (AMI) LargeArray. These objects were previously observed with the AMI Small Array to have an excessof emission at microwave frequencies relative to lower frequency radio data. In L675 we finda flat spectrum compact radio counterpart to the 850 m m emission seen with SCUBA andsuggest that it is cm-wave emission from a previously unknown deeply embedded young pro-tostar. In the case of L1246 the cm-wave emission is spatially correlated with 8 m m emissionseen with Spitzer . Since the MIR emission is present only in
Spitzer band 4 we suggest thatit arises from a population of PAH molecules, which also give rise to the cm-wave emissionthrough spinning dust emission.
Key words:
Radiation mechanisms: general – ISM: general – ISM: clouds – stars: formation
The complete characterization of microwave emission from spin-ning dust grains is a key question in both astrophysics and cosmol-ogy. It probes a region of the electromagnetic spectrum where anumber of different astrophysical disciplines overlap. It is impor-tant for CMB observations in order to correctly characterise thecontaminating foreground emission; for star and planetary forma-tion it is important because it potentially probes a regime of grainsizes that is not otherwise easily observable.Although a number of objects have now been found to exhibitanomalous microwave emission, attributed to spinning dust, it isstill unclear what differentiates those objects from the many otherseemingly similar targets that do not show the excess. In the spe- ⋆ We request that any reference to this paper cites “AMI Consortium: Scaifeet al. 2010”† E-mail: [email protected] cific case of dark clouds the recent AMI sample (AMI Consortium:Scaife et al. 2009; hereinafter Paper I) of fourteen Lynds Dark Neb-ulae found an excess in only five.It has been suggested that cm-wave emission from spinningdust is emitted by a population of ultra-small grains (Draine &Lazarian 1998). These ultra-small grains are thought to exist mainlyin the form of single polycyclic aromatic hydrocarbon (PAH)molecules. PAH molecules are generally detected through their nar-row line emission features in the MIR. For these emission featuresto be observed the PAH molecules must be exposed to a strongsource of UV flux. Since this flux is generally absent in the caseof dark clouds, the microwave emission from the rotation of PAHmolecules may be the only way to study the very small grain pop-ulation in these objects.It is also known that radio continuum emission in dark cloudsmay arise from ionized gas associated with a stellar outflow. Whena luminous star is present this arises either as the result of a com-pact H II region or an ionized stellar wind. In the case of very young c (cid:13) Scaife et al. low luminosity stars radio continuum emission may be also be de-tected. In this instance it is generally attributed to the presence ofa partially ionized (0 . x e .
35; Bacciotti & Eisl¨offel 1999)stellar wind (Wright & Barlow 1975; Panagia & Felli 1975), orpossibly a neutral wind which has been shock-ionized further fromthe central source by impacting on a dense obstacle (Curiel et al.1989).In this paper we present follow-up observations of the fiveAMI Small Array (SA) spinning dust detections (Paper I) at higherresolution with the AMI Large Array (LA) over the same frequencyrange. All co-ordinates in this paper are J2000.0.
AMI comprises two synthesis arrays, one of ten 3.7 m antennas(SA) and one of eight 13 m antennas (LA), both sited at Lord’sBridge, Cambridge (AMI Consortium: Zwart et al. 2008). Thetelescope observes in the band 13.5–17.9 GHz from which eight0.75 GHz bandwidth channels are synthesized. In practice, the twolowest frequency channels (1 & 2) are not generally used due to alower response in this frequency range and interference from geo-stationary satellites.Observations of five Lynds dark nebulae selected from theoriginal AMI SA sample were made in 2009 February–March us-ing the AMI LA. The co-ordinates of these fields are listed in Ta-ble 1 along with the size of the AMI LA synthesized beam towardseach object and the r.m.s. noise measured outside the primary beamon the CLEANed maps. We note that the AMI LA observation ofL1246 is towards the north–east of this cloud where anomalousemission was detected by the AMI SA and does not cover the samearea as the original SCUBA observation.Data reduction was performed using the local software tool
REDUCE , see Paper I for more details. Flux calibration was per-formed using short observations of 3C286 near the beginning andend of each run. We assumed I+Q flux densities for this sourcein the AMI LA channels consistent with the frequency dependentmodel of Baars et al. (1977), ≃ . ◦ for chan-nels 4–7, and 10 ◦ for channels 3 and 8. The FWHM of the primarybeam of the AMI LA is ≈ ′ at 16 GHz.Reduced data were imaged using the AIPS data package. CLEAN deconvolution was performed using the task
IMAGR whichapplies a differential primary beam correction to the individual fre-quency channels to produce the combined frequency image. De-convolved maps were made from both the combined channel set,see Fig. 1, and for individual channels. The broad spectral cover-age of AMI allows a representation of the spectrum between 14.3and 17.9 GHz to be made independently of other telescopes andin what follows we use the convention: S (cid:181) n − a , where S is fluxdensity, n is frequency and a is the spectral index. All errors arequoted to 1 s . Table 1.
AMI LA Lynds Dark Nebulae. Column [1] Name of cloud, [2]Right Ascension, [3] Declination, [4] AMI LA synthesized beam FWHMmajor axis, [5] AMI LA synthesized beam FWHN minor axis, and [6] r.m.s.noise fluctuations on the combined channel map.Name RA Dec
D q maj
D q min s rms (J2000) (J2000) (arcsec) (arcsec) ( m Jybm )L675 19 23 52.6 11 07 39 49.9 27.4 35L944 21 17 40.8 43 18 08 36.5 31.2 31L1103 21 42 10.2 56 43 44 32.0 26.3 25L1111 21 40 27.1 57 48 10 39.4 30.8 29L1246 23 25 30.1 63 38 30 31.2 26.9 25
Figure 2.
L675 Source A: data points are flux densities from AMI LA chan-nels 3–8. The best-fit spectral index of a = . ± .
36 is shown as a dashedline.
The AMI LA observations of L675 show two obvious re-gions of compact emission, see Fig. 1. The first of these, slightlyoffset from the pointing centre, is coincident with both the peakof the AMI SA emission and also the compact emission seen at850 m m by the SCUBA instrument (Visser et al. 2001; 2002). Wedenote this source “A” (19 h m . s + ◦ ′ ′′ ). The second,just outside the LA primary beam FWHM to the north-east, is co-incident with the probable extragalactic point source identified as“B” (19 h m . s + ◦ ′ ′′ ) in the original AMI SA observa-tions (Paper I).Source A is completely unresolved by the AMI LA and showsa flat spectrum across the AMI band, a . . = . ± .
36, consis-tent with free–free emission, see Fig. 2. This spectral index differsconsiderably from that measured by AMI SA. This is because theLA is not sensitive to the large scale emission seen with the SA. In-deed it seems likely that the emission seen by the two arrays arisesfrom completely different sources.
L944:
The original AMI SA observations of L944 revealeda compact region of emission to the north of the cloud, coincidentwith one side of the protostellar outflow. AMI LA observations, seeFig. 1, reveal this emission arises not from a point-like object butrather from a diffuse region of emission, the peak of which occursat 21 h m . s + ◦ ′ ′′ . We estimate the flux spectrum by in-tegrating the flux density from the primary beam corrected channelmaps within a two arcminute radius of the LA pointing centre. Thisshows a steeply rising spectrum with a . . = − . ± .
5. This isconsistent with that found from the AMI SA data, however this cor-respondence is not meaningful as the low signal to noise in the SAdata precludes a precise estimate. The flux density found towardsthis region in the LA map is only marginally lower than that foundfrom the comparatively coarser resolution SA map. This implies c (cid:13) , 1–5 MI LA spinning dust observations L675 L944L1103 L1111L1246
Figure 1.
AMI LA combined channel data is shown as greyscale in units of mJy/beam, grey conrtours at − , − , ± , ± s and black contours at 3, 6, 12, 24 s etc. SCUBA 850 m m data is shown as red contours with levels as in Visser et al. (2001; 2002) for the all clouds except L1246. The AMI LA observation ofL1246 does not cover the region observed by SCUBA. AMI SA data is shown as blue contours with levels as in Paper I. The AMI LA primary beam FWHMis shown as a circle and the synthesized beam as a filled ellipse in the bottom left corner.c (cid:13) , 1–5 Scaife et al.
Table 2.
Integrated flux densities in mJy for AMI LA observations of L675, L944 and L1246. Errors are calculated as s = p ( . S ) + s , where s rms isthe r.m.s. noise in the individual channel map. Freq. (GHz)Name 14.3 15.0 15.7 16.4 17.2 17.9 a L675 2.74 ± .
34 2.24 ± .
12 2.26 ± .
12 2.12 ± .
11 2.42 ± .
14 2.22 ± . + . ± ± .
11 2.40 ± .
13 2.89 ± .
15 2.86 ± .
15 - − . ± . ± .
18 0.55 ± .
03 0.62 ± .
03 0.58 ± .
04 - − . ± . that the emission comes not from one smooth extended region thatis partially resolved out by the LA baselines, but from a collectionof smaller fragments or filaments. These fragments are unresolvedby either array, although the granularity becomes more evident inthe higher frequency channels of the LA. The amount of flux lostin channels 5 to 8 relative to channel 4 is significantly smaller thanwould be expected from a Gaussian source of similar dimensions. L1103 and L1111:
AMI LA observations of L1103 andL1111 do not show any distinct regions of compact cm-wave emis-sion. The diffuse patches of low level emission present within theprimary beam towards both sources are indicative of larger scalestructures which have been resolved out by the synthesized beam.We can provide an estimate of the flux density seen towards theseobjects with the LA by fitting and removing a tilted plane baselevelat the primary beam FWHM. From the combined channel data thisgives S = . ± . S = . ± . L1246:
AMI SA observations towards L1246 did not showany excess emission coincident with the SCUBA identification ofthe dark cloud, but did reveal a region of emission ≈ ′ to the north–east of the cloud, in a region not covered by the SCUBA map,which had no counterpart in the lower frequency observations. AMILA observations of this NE region show an arc of emission (peak:23 h m . s + ◦ ′ ′′ ), see Fig. 1. We assess the spectral be-haviour of this object in two ways. Firstly, we estimate the fluxdensity of the arc itself. We fit and remove a tilted plane baselevelwithin an irregular polygon drawn around the object and integratethe remaining flux. Secondly, we fit a tilted plane baselevel to acircle at the primary beam FWHM and integrate all the flux abovethis baselevel within that radius. Both methods give consistent re-sults, as might be expected since the primary beam is relativelyempty otherwise. From the first method we find a spectral index a . . = − . ± .
87, and from the second a . . = − . ± . L675 and L1246 have archival
Spitzer
IRAC data, which shows inboth cases a significant amount of emission in Band 4 (6.4–9.4 m m)and very little in the other three (3.2–3.9, 4.0–5.0 and 4.9–6.4 m m,respectively). In the case of L675 this emission is present on a verylarge scale, see Fig. 3. The emission seen at 16 GHz with the AMISA appears on a similar scale, however the small field of view of the Spitzer data precludes a more detailed comparison. L1246 showsan arc of emission at 16 GHz which is also evident in
Spitzer
IRACBand 4, see Fig. 4. This emission is again not present in Bands 1–3.
Figure 3.
L675: AMI LA combined channel data is shown as white contoursat 3, 6, 12 s etc. Spitzer
Band 4 data is shown as greyscale in MJy/sr, andis saturated at both ends to emphasise the structure present. AMI SA data isshown as blue contours as in Fig. 1.
Figure 4.
L1246: AMI LA combined channel data is shown as white con-tours at 1, 2, 4, 8 s etc. Spitzer
Band 4 data is shown as greyscale in MJy/sr,saturated at both ends of the scale to emphasise the structure present. TheAMI LA primary beam is shown as a circle and the synthesized beam as afilled ellipse in the bottom left corner.
In Band 4 it is present as an arc, coincident with that seen at 16 GHzin the AMI LA data.
Spitzer
Band 4 contains two of the PAH emission lines, in-cluding the strongest (7.7 m m). Of the three other Spitzer bandsonly Band 1 contains an emission line (3.3 m m) and for ionizedPAHs this line is expected to be significantly weaker. It is probabletherefore that the MIR correlated cm-wave data seen in the AMImaps is a consequence of spinning dust emission from a popula- c (cid:13) , 1–5 MI LA spinning dust observations tion of ionized PAH molecules. Neutral PAH molecules do not ingeneral possess a permanent dipole moment and are therefore notexpected to have rotational emission (Tielens 2008). This emission,the mechanism of which is described in detail by Draine & Lazar-ian (1998), arises from the intrinsic dipole moments of small dustgrains, most likely to be PAH molecules, which emit power whenthey rotate. This rotation has a variety of contributing factors, therelative importance of which varies with grain environment. How-ever, in the majority of cases excitation through collision with ionsis predominant.In the case of L675A, we must consider the possibility that weare observing a coincidental extragalactic radio source. Using theextended 9C survey 15 GHz source counts (Waldram et al. 2009),where n ( S ) = ( S / Jy ) − . Jy − sr − , the probability that a sourcewith flux density greater than 2 mJy lies within the FWHM of theAMI LA primary beam is 0 .
12, and only 0.01 within the SCUBAfield. It is likely therefore that the radio source L675A is associatedwith the SCUBA core.A further question is whether the cm-wave emission might beexplained by thermal (Planckian) dust emission. A single greybodyspectrum with a dust temperature, T d ≈
27 K, might be used to ex-plain the LA flux density, however it would require a b of 0.6. Sucha value would be unusual even for objects known to possess flat-tened dust tails, such as protoplanetary disks. This simple fit alsoneglects the flux lost by the AMI LA baseline distribution. SA ob-servations have already shown this source to possess a significantamount of extended emission which would make this scenario evenmore unlikely.The presence of a neutral or partially ionized wind from anoutflow source that has been shocked through encountering a denseobstacle (Torrelles et al. 1985; Rodr´ıguez et al. 1986) is used tounderstand the spectral indices seen towards exciting sources inthe radio regime (Curiel et al. 1990; Cabrit & Bertout 1992). Thismodel allows a spectral index range of 0 . a = − . t = . − , we cancalculate that the AMI flux densities towards L675A are consistentwith a mass loss of 3 . × − M ⊙ yr − . A mass loss such as thisimplies a mechanical luminosity from the wind of L mech ≈ . ⊙ ,comparable to the values found by Curiel et al for L1448.The nature of the emission seen towards L944 with the AMILA is uncertain. The spectral index of this emission is consis-tent with spinning dust emission or alternatively the optically thickcomponent of free–free spectrum. Such a free–free spectrum mightbe exhibited at 16 GHz by ultra-compact H II regions. However aturn-over frequency above 16 GHz would have an extremely highmass and should therefore be obvious in sub-mm observations. Thisneeds to be confirmed by either higher radio frequency measure-ments in order to measure the optically thin region of the spectrumand the turn-over, or sub-mm measurements to place constraints onthe mass of such a region.In conclusion, we have used the AMI LA to observe a sampleof five Lynds Dark Nebulae selected as candidates for spinning dustemission from the AMI SA sample of Lynds Dark Nebulae (PaperI). Towards two of these clouds (L1103 and L1111) we detect onlypatchy diffuse emission characteristic of the presence of a largerstructure which has been mostly resolved out. Towards L675 we have observed flat spectrum compact cm-wave emission coincident with the SCUBA 850 m m emission fromthe same region. These characteristics suggest that this source isassociated with a stellar wind from a deeply embedded young pro-tostar.We detect extended cm-wave emission to the North of theL944 SMM-1 protostar which displays spectral behaviour consis-tent with either spinning dust, or alternatively a collection of ultra-compact H II regions.L1246 shows an arc of cm-wave emission which is coincidentwith emission seen in Spitzer
Band 4. We suggest that this is anexample of emission from a population of PAH molecules, seen inemission lines in the
Spitzer data, and emission as a consequenceof rapid rotation of the molecules in the cm-wave data.
We thank the staff of the Lord’s Bridge Observatory for their in-valuable assistance in the commissioning and operation of AMI.AMI is supported by Cambridge University and the STFC. NH-W,CR-G, TWS, TMOF, MO and MLD acknowledge the support ofPPARC/STFC studentships. This work is based in part on archivaldata obtained with the Spitzer Space Telescope, which is operatedby the Jet Propulsion Laboratory, California Institute of Technol-ogy under a contract with NASA. Support for this work was pro-vided by an award issued by JPL/Caltech.
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