J/ApJ/919/121 Planetary nebulae with LOFAR 120-168MHz obs. (Hajduk+, 2021) ================================================================================ Evidence for cold plasma in planetary nebulae from radio observations with the LOw Frequency ARray (LOFAR). Hajduk M., Haverkorn M., Shimwell T., Olech M., Callingham J.R., Vedantham H.K., White G.J., Iacobelli M., Drabent A. =2021ApJ...919..121H ================================================================================ ADC_Keywords: Planetary nebulae; Radio continuum Keywords: Planetary nebulae ; Radio continuum emission ; Post-asymptotic giant branch stars ; Interstellar dust extinction Abstract: We present observations of planetary nebulae with the LOw Frequency ARray (LOFAR) between 120 and 168MHz. The images show thermal free-free emission from the nebular shells. We have determined the electron temperatures for spatially resolved, optically thick nebulae. These temperatures are 20%-60% lower than those estimated from collisionally excited optical emission lines. This strongly supports the existence of a cold plasma component, which co- exists with hot plasma in planetary nebulae. This cold plasma does not contribute to the collisionally excited lines, but does contribute to recombination lines and radio flux. Neither of the plasma components are spatially resolved in our images, although we infer that the cold plasma extends to the outer radii of planetary nebulae. However, more cold plasma appears to exist at smaller radii. The presence of cold plasma should be taken into account in modeling of radio emission of planetary nebulae. Modelling of radio emission usually uses electron temperatures calculated from collisionally excited optical and/or infrared lines. This may lead to an underestimate of the ionized mass and an overestimate of the extinction correction from planetary nebulae when derived from the radio flux alone. The correction improves the consistency of extinction derived from the radio fluxes when compared to estimates from the Balmer decrement flux ratios. Description: the LOw Frequency ARray (LOFAR) is a radio interferometer which consists of 52 stations distributed in Europe. The Netherlands host 24 core and 14 remote stations operating at the shortest baselines. The remaining 14 stations are located in other countries and provide the longest baselines. Each single station consists of a set of low-band and high-band antennas observing in the 30-80 and 110-240 frequency ranges, respectively. We used the radio continuum 120-168MHz images (central frequency of 144MHz) of planetary nebulae (PNe) collected by the LOFAR Two-Metre Sky Survey (LoTSS; Shimwell+ 2019, J/A+A/622/A1). The survey uses only the data from core and remote stations. The survey provides low- and high-resolution images with the full width at half maximum of the restoring beam being 20 and 6", respectively. The median positional accuracy of the high-resolution images is 0.2", though it may range from 0.1" to 4.8" for individual fields. The LoTSS observations and data processing are still ongoing. We included observations which were processed before 2021 April. This largely overlapped with the upcoming LoTSS-DR2 (Shimwell+ 2022, J/A+A/659/A1). See Section 3. We selected 165 PNe in the observed part of sky using the SIMBAD database and the catalog by Parker+ (2016JPhCS.728c2008P). Out of them, 30 were detected (see Table 1). File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 60 30 The flux densities and diameters taken from Frew+ (2016MNRAS.455.1459F) of PNe detected in the LOFAR Two-Metre Sky Survey (LoTSS) table3.dat 38 135 Upper limits for nebular flux densities at 144MHz and corresponding brightness temperatures -------------------------------------------------------------------------------- See also: V/127 : MASH Catalogues of Planetary Nebulae (Parker+ 2006-2008) J/ApJS/117/361 : PNe in NRAO VLA Sky Survey (Condon+ 1998) J/A+A/373/1032 : Radio emission from planetary nebulae (Siodmiak+, 2001) J/MNRAS/428/3443 : PNe and HII regions plasma diagnostics (McNabb+, 2013) J/A+A/598/A104 : LOFAR Two-metre Sky Survey (Shimwell+, 2017) J/A+A/622/A1 : LOFAR Two-metre Sky Survey DR1 source cat. (Shimwell+, 2019) J/MNRAS/503/2887 : PNe angular diameters from SED modeling (Bojicic+, 2021) J/A+A/648/A104 : LOFAR LBA Sky Survey. I. (de Gasperin+, 2021) J/A+A/659/A1 : LOFAR Two-metre Sky Survey (LoTSS) DR2 (Shimwell+, 2022) http://lofar-surveys.org/lotss-tier1.html : LOFAR surveys current status http://202.189.117.101:8999/gpne/dbMainPage.php : HASH PN database 4.6 Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 11 A11 --- Name PN name 13- 18 F6.2 mJy F144MHz [0.95/191] Flux density at 144MHz 20- 24 F5.2 mJy e_F144MHz [0.2/69] F144MHz uncertainty 26- 30 F5.1 arcsec dDiam1 [5.7/185.9]? Deconvolved diameter 1, {Theta}_d_ (1) (2) 32- 36 F5.1 arcsec dDiam2 [3.6/182.8]? Deconvolved diameter 2, {Theta}_d_ (1) (2) 38- 42 F5.2 arcsec cDiam [8/45.8]? Corrected diameter, {Theta} (1) 44- 48 F5.2 arcsec e_cDiam [0.57/11]? cDiam uncertainty (1) 50- 54 F5.1 arcsec ODiam1 [2.7/208]? Optical diameter 1 56- 60 F5.1 arcsec ODiam2 [2.7/202]? Optical diameter 2 -------------------------------------------------------------------------------- Note (1): The deconvolved and corrected diameters are not given for unresolved PNe. Note (2): Large PNe were not fitted with a Gaussian and their diameters {Theta}_d_ refer to the size, which exceeded 3{sigma} (see text). -------------------------------------------------------------------------------- Byte-by-byte Description of file: table3.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 25 A25 --- Name Planetary nebula name 27- 32 F6.3 mJy/beam rms [0.27/10.2] Upper limit of nebular flux density at 144MHz (3xrms) (1) 34- 38 I5 K Tb [639/23976] Brightness temperature (T_B_) (1) -------------------------------------------------------------------------------- Note (1): The median root mean square (rms) is about 700uJy/beam. A 3rms upper limit of 2.1mJy/beam at 144MHz corresponds to a brightness temperature of about 4800K for a 6" beam. -------------------------------------------------------------------------------- Acknowledgements: Marcin Hajduk [marcin.hajduk at uwm.edu.pl] History: From electronic version of the journal ================================================================================ (End) Emmanuelle Perret [CDS] 31-Jan-2023