Observation of Carbon Radio Recombination Lines Towards Dust Cloud L1407 at decameter wavelengths.

Hamburg, 16-19 September 2008 1

Observation of Carbon Radio Recombination Lines Towards Dust Cloud L1407 at decameter

wavelengths.

S. V. Stepkin, A. A. Konovalenko, D. V. Mukha.

Institute of Radio Astronomy, Kharkiv, Ukraine


Abstract

Extremely low frequency radio recombination lines (RRLs) could be used as effective means of the low density partially ionized interstellar medium diagnostic. Nevertheless, low intensities of the features and high level of interferences makes such investigations difficult. The best-studied object with extremely low frequency carbon RRLs is the medium in the direction of Cassiopeia A, and positive detections of such phenomena are still not too numerous. It is possible to improve efficiency of the low frequency RRLs studies by increasing frequency band of experiments. In our case of very high atom quantum states n is much bigger than delta n. So adjacent lines can be considered as equivalent and we could fold individual transitions in order to improve measurement sensitivity. Such approach have been used in the course of observations towards dust cloud L1407 with radio telescope UTR-2 near 25 and 26 MHz that have yielded the detection of a series of carbon RRLs.


n>600, < 30 MHzTL/TC =-L

*bnn=f(Ne, Te, S, ME)

L*=2*106 Ne

2 S/Te2. 5

D=L V/c

p~Ne n5. 2

R~n5. 8

The distance between adjacent features L~3/n

In 20 … 30 MHz band there are ~ 90 lines.


Carbon in the ISM

Carbon is the most abundant element with ionization potential less then ionization potential of hydrogen (C/H=3.710-4, Ec=11.2 eV, EH=13.6 eV);

It is almost completely ionized in the interstellar rarified gas by ultraviolet photons with 912 Å<<1100 Å;


Carbon in the ISM (2)

• It plays important role in the thermostatic processes in HI clouds due to fine structure transition 2P3/2 — 2P1/2

with =158 ;

• The most of the interstellar molecules contain carbon ;

• Carbon plays significant roles in the chemical reactions in the interstellar gas.


Dust Dark Cloud L1407

It is spherical isolated cloud with diameter ~ 16

In the vicinity of L1407 there are three bright B1 stars

Galactic coordinates l=151.44 b=3.98

Observed lines 12CO, 13CO, and 6cm H2CO


Cloud Properties

• Distance 170 pc

• VLSR 1.5 km/s

• Peak TK 11 K

• Diameter 1.1 pc

• Average density 2.4 x 10-3 cm-3

• Mass 57 M (of the Sun)


Observations, 1987 – 1989

UTR-2 North-South arm + East-West armObserved lines corresponded n=635-643 (analysis band ~ 20 kHz) (Frequencies near 25 MHz, integration 572 h)Relative intensity -0.0007 0.0003Observed width 1.5 0.5 kHzVLSR -10 5 km/s


Averaged spectrum (1987 – 1989), integration 572 h

*10-4

Radial velocity, km/s


Observations, 2002-2003

UTR-2 North-South armObserved lines corresponded n=627-636 (analysis band ~ 1 MHz) (Frequencies near 26 MHz, integration 172 h)


Averaged spectrum C627 - C636 (2002 –2003), integration 172 h


Observations, 2008

UTR-2 East-West armObserved lines corresponded n=636-645 (analysis band ~ 1 MHz) (Frequencies near 25 MHz, integration 53 h)


Averaged spectrum C636 - C645 (2008), integration 53 h


Averaged spectrum (2002, 2003, 2008), integration 225 h


Galactic Plane, l=140, b=0


Conclusions

Observations of extremely low frequency RRLs is promising means of the ISM diagnostics.

Improving of spatial resolution and antenna efficiency will open new opportunities in this field of space radio spectroscopy. Mapping of the Galactic Plane with carbon low frequency RRLs is especially interesting and can be among the tasks for the future investigation with LOFAR.


References

1. Konovalenko, A. A., Sodin, L. G. The 26.13 MHz absorption line in the direction of Cassiopeia A. Nature 294, 135 (1981).

2. Snell L. Ronald. A study of nine interstellar dark clouds. Astrophys. J. Supplement Series, 45, 121–175 (1981).

3. E.A. Abramenkov. Decameter observations of HII regions in the galactic disc, 147l153. Astron. Zh., 62, 1057-1064, 1985.


References (2)

4. А.А. Голынкин, А.А. Коноваленко. Обнаружение рекомбинационных линий углерода на декаметровых волнах в пылевом облаке L1407. Письма в АЖ. т. 17, №1, 23-29 (1991)..

5. Gordon, M. A., Sorochenko, R. L. Radio Recombination Lines: Their Physics and Astronomical Application. , (Kluwer, 2002).

6. Stepkin, S. V., Konovalenko, A. A. , Kantharia, N. G., & Udaya Shankar, N. Radio recombination lines from the largest bound atoms in space. Mon. Not. R. Astron. Soc. 374, 852-856 (2007).

Observation of Carbon Radio Recombination Lines Towards Dust Cloud L1407 at decameter wavelengths.

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