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.
<Astrophys. J., 919, 121 (2021)>
=2021ApJ...919..121H 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 2016JPhCS.728c2008P).
Out of them, 30 were detected (see Table 1).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 60 30 The flux densities and diameters taken from
Frew+ (2016MNRAS.455.1459F 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
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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
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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,
Θd (1) (2)
32- 36 F5.1 arcsec dDiam2 [3.6/182.8]? Deconvolved diameter 2,
Θd (1) (2)
38- 42 F5.2 arcsec cDiam [8/45.8]? Corrected diameter, Θ (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
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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
Θd refer to the size, which exceeded 3σ (see text).
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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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 (TB) (1)
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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.
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Acknowledgements:
Marcin Hajduk [marcin.hajduk at uwm.edu.pl]
History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 31-Jan-2023