Search for Compact Extragalactic Radio Sources Near Massive(3)

We have used the Very Large Array to search for compact milliarcsecond-size radio sources near methanol masers in high-mass star-forming regions. Such sources are required for Very Long Baseline Interferometry phase-referencing observations. We conducted p

unresolvedinC-band.Theseobservationsweredoneaslow-priorityobservationsinsmallgapsintheVLAscheduleintheA-orB-arrayatU-bandon2004September26,27,28,October29,and2005May1,10,and26,respectively.WechoseU-bandasitistheVLAfrequencythatisclosesttothatofthe12.2GHzmasers.Theobservationsofeachobjectinvolved6short(1minduration)scans,spreadoverarangeintimeforbetteruv-coverage.Tominimizethephaseerrorsarisingfromtemporalchangesintheatmosphericphasepaths,weobservedastandardVLAcalibratorbeforeandaftereachtargetsource.Typicalmaprmsnoiselevelswereabout0.2mJy.Thisallowedusdeterminepositionsaccuratetobetterthan0.1arcsec.ItisimportanttoreachthatprecisionforVLBIastrometryinordertominimizesecond-orderpositionalerrorsandimproveimagequality.

3.Results

Wedetectedabout92ofthe234candidatesourceswith uxdensitiesof5mJyatC-bandwith attishorfallingspectralindexes.ThesearelikelycompactextragalacticsourcesassociatedwithAGNactivity.

InTable2welistweaksources<5mJyatC-bandobservedintheC-con guration.Thesearetooweakforcurrentastrometricobservations.

InTable3welistthesourcesthatwereclearlyresolved.ThesesourcesareprobablymostlyHIIregions,PNe,andSNRs.Whilesomeultra-andhyper-compactHIIregionsmaynotberesolvedbytheVLA,theyshouldhave atorrisingspectraatcentimeterwavelengths(Wood&Churchwell1989;Kurtzetal.1994),thatcanbeeasilyrecognizedinourdual-frequencyobservations.AlthoughSNRshavefallingspectra,theyshouldberesolvedduetotheirlargersize(Green2004).ThesourcesdetectedaC-band,butnotatU-band,mightbeusedascalibratorsatlowerfrequency,e.g.,forVLBIobservationsof6.7GHzCH3OHmasers,and/or1.6and6.0GHzOHmasers.

Table4liststheparametersofthe92sourcesthatareunresolvedatC-band,have atorfallingspectralindexes,andarepotentiallystrongenoughtobecalibrationsourcesforVLBIphase-referencedobservations.TheirimagesareshowninFigure1whichisavailableintheelectronicversionofthispaper.Exceptforafewsources,consistingoftwocomponents,mostofthesourcesappeardominatedbyasinglecompactcomponent.Mostofthemcontainover85%oftheir uxdensityinanunresolvedcomponentatC-and/orU-bandandarelikelycompactextragalacticsources.EventhoughweselectedsourceswithourC-con gurationobservationsthatwerenearlyunresolvedandhad>5mJyatC-band,whenobservedwiththemoreextendedA-orB-con gurationssomewerenotdetected.Sincethesemightcontain