influence of gamma-ray emission on the isotopic composition of clouds in the interstellar medium
Post on 30-Sep-2016
Embed Size (px)
ISSN 1063-7737, Astronomy Letters, 2012, Vol. 38, No. 6, pp. 364379. c Pleiades Publishing, Inc., 2012.Original Russian Text c V.V. Klimenko, A.V. Ivanchik, D.A. Varshalovich, A.G. Pavlov, 2012, published in Pisma v Astronomicheski Zhurnal, 2012, Vol. 38, No. 6, pp. 414430.
Inuence of Gamma-Ray Emission on the Isotopic Compositionof Clouds in the Interstellar Medium
V. V. Klimenko1, 2, A. V. Ivanchik1,2*, D. A. Varshalovich1,2, and A. G. Pavlov2
1Ioe PhysicalTechnical Institute, ul. Politekhnicheskaya 26, St. Petersburg, 194021 Russia2St. Petersburg State Polytechnical University, ul. Politekhnicheskaya 29, St. Petersburg, 195251 Russia
Received December 19, 2011
AbstractWe investigate one mechanism of the change in the isotopic composition of cosmologicallydistant clouds of interstellar gas whose matter was subjected only slightly to star formation processes.According to the standard cosmological model, the isotopic composition of the gas in such clouds wasformed at the epoch of Big Bang nucleosynthesis and is determined only by the baryon density in theUniverse. The dispersion in the available cloud composition observations exceeds the errors of individualmeasurements. This may indicate that there are mechanisms of the change in the composition of matterin the Universe after the completion of Big Bang nucleosynthesis. We have calculated the destruction andproduction rates of light isotopes (D, 3He, 4He) under the inuence of photonuclear reactions triggeredby the gamma-ray emission from active galactic nuclei (AGNs). We investigate the destruction andproduction of light elements depending on the spectral characteristics of the gamma-ray emission. Weshow that in comparison with previous works, taking into account the inuence of spectral hardness on thephotonuclear reaction rates can increase the characteristic radii of inuence of the gamma-ray emissionfrom AGNs by a factor of 28. The high gamma-ray luminosities of AGNs observed in recent years increasethe previous estimates of the characteristic radii by two orders of magnitude. This may suggest that theinuence of the emission from AGNs on the change in the composition of the medium in the immediateneighborhood (the parent galaxy) has been underestimated.
Keywords: cosmology, primordial composition of matter, gamma-ray emission.
The relative abundance of the light elements (H,D, 3He, 4He, 6Li, 7Li) produced at the epoch ofBig Bang nucleosynthesis is one of the criteria fortesting the standard cosmological model. Accordingto this model, Big Bang nucleosynthesis occurred inthe early Universe: at times from several minutes,when the Universe was extremely hot (T > 109 K),to half an hour, when the temperature and densityof the medium dropped to values at which the nu-clear reactions no longer proceeded. Subsequently,the composition of the interstellar and intergalacticmedium changed mainly under the inuence of astra-tion and radioactive decay. Therefore, by studying theisotopic composition of distant clouds of interstellargas (z 23), where the star formation processeshad not yet strongly aected the relative elementalabundances, their abundances at the completion timeof Big Bang nucleosynthesis can be determined.
One of the most important aspects of such ananalysis is the abundance of deuterium. In compar-ison with other elements produced at the epoch of BigBang nucleosynthesis, deuterium has the simplestnuclear structure and is most sensitive to a changein the baryon density bone of the key parametersof the standard cosmological model. The D/H ratio inthe interstellar medium of galaxies can be calculatedfrom the absorption by neutral gas of light emitted bydistant quasars (QSO) by comparing the intensitiesof hydrogen absorption lines with those of deuteriumones. When the primordial D/H ratio is estimated,deuterium is commonly assumed to be only destroyedin the course of stellar evolution. Therefore, any D/Hestimate is considered only as a lower limit for theprimordial D/H ratio.
Some of the rst deuterium abundance measure-ments in clouds of interstellar gas at high redshiftsgave large D/H ratios, 104 (Webb et al. 1997;Rugers and Hogan 1996), which was inconsistentwith the predictions of the standard model. Anoverview of the possible mechanisms of the change
INFLUENCE OF GAMMA-RAY EMISSION 365
in the abundances of light elements in clouds ofinterstellar gas relative to their primordial values wasprovided by Gnedin and Ostriker (1992). However,the new deuterium abundance observations (Burlesand Tytler 1998) disproving the previous ones werein agreement with the theory, and the question aboutthe change in abundance was essentially lifted. Thecurrently available light-element abundance datataken from Steigman (2007) are presented in Table 1.Eleven absorption systems of atomic deuterium andhydrogen have been detected in the last ten years, andthe D/H ratio has been measured in them with a highaccuracy. The derived values are consistent, in orderof magnitude, with the conclusions of the standardmodel, but there are some problems. At present, onlyseven systems in which the deuterium abundancecan be determined with good condence from ananalysis of absorption lines (Pettini et al. 2008,D/H = 2.82 0.20 105) are used to estimate theD/H ratio. However, even for this sample the 1scatter of D/H values was about 20%, which isconsiderably higher than the observational errors ofindividual measurements.
It is believed that the scatter of deuterium abun-dances in such clouds can be provided by both starformation processes and deuterium depletion ontodust grains (Linsky et al. 2006). The interstellar gasthat was subjected only slightly to chemical enrich-ment, as is suggested by the low relative abundance ofheavy elements, O/H < 1/10 of the value measuredin the Solar system, was studied in these seven sys-tems. In the model of galactic chemical evolution,the decrease in the D/H ratio for such clouds is neg-ligible, which was theoretically shown by Akermanet al. (2005), Prantzos and Ishimaru (2001), andRomano et al. (2006). Therefore, the large dispersionin the experimental data is most likely explained byan underestimation of the statistical and systematicerrors (Steigman 2007; Pettini et al. 2008). Apartfrom the standard theory of Big Bang nucleosynthe-sis, the dispersion in the light-element abundanceobservations is associated with the evaporation ofprimordial black holes or the decay of long-lived X-particles. The high-energy hadrons and photonsbeing produced during the decay of X-particles de-stroy the light elements synthesized in Big Bangnucleosynthesis. Eective 4He destruction and D,T, 3He production can occur as a consequence ofthis process. These processes were investigated byZeldovich et al. (1977), Kawasaki et al. (2005), andCarr et al. (2010). Stringent constraints were placedon the primordial abundance of the long-lived exoticX-particles.
In this paper, we investigate one mechanism ofthe change in the relative composition of the medium
Table 1. Primordial isotopic composition of matter in theUniverse
Element abundancerelative to hydrogena Primordial value
XD nD/nH 2.68 105X3He n3He/nH 1.06 105X4He n4He/nH 7.90 102X7Li n7Li/nH 4.30 1010
a n is the element number density in the medium.b The data were taken from Steigman (2007). The abundanceswere reconciledwith the baryon density B obtained by analyzingthe CMB anisotropy data.
that was previously considered by Boyd et al. (1989)and Gnedin and Ostriker (1992). Hard X-ray andgamma-ray emission is present in the spectrum ofactive galactic nuclei (AGNs). Propagating in theUniverse, the high-energy gamma emission interactwith the interstellar and intergalactic matter. Thephotodisintegration of 4He with the production of Dcan be compared with the photodisintegration of D.This can cause both a decrease and an increase inthe relative abundance of deuterium in some regionsof interstellar gas. The magnitude of this eect de-pends on the initial composition of the medium (theabundance of 4He in a cloud is greater than that of Dby three orders of magnitude), the spectral hardnessof the gamma-ray emission characterized by the pa-rameter (see Eq. (3)), and its ux.
Boyd et al. (1989) estimated this eect using thegalactic center of NGC 4151 as an example. The au-thors showed that the gamma-ray source associatedwith the galactic center could cause 4He photodisin-tegration and signicant D production. However, theradius of inuence of this process is very small anddoes not exceed 10 light-years in the case under con-sideration. The authors conclude that, given the rarityof gamma-ray sources in the Galaxy, the change inthe isotopic composition of the Galactic medium is,in general, unlikely. Nevertheless, Casse et al. (1999)pointed out that blazars could aect the compositionof the matter accreting onto a black hole. When out-owing from the blazars central region, this mattercan condense into intergalactic clouds. The authorsalso note that the nal D abundance is sensitive tothe spectral index of the quasar emission. However,the authors made no quantitative estimations.
The goal of this paper is to consider the mech-anism of element photodisintegration in a mediumby the emission from quasars by taking into ac-count various spectral indices of emission and the
ASTRONOMY LETTERS Vol. 38 No. 6 2012
366 KLIMENKO et al.
0 10 20 30 40 50 60 70 80 90E, MeV
Hara et al. (2003)Birenbaum et al. (1985)Skopik et al. (1974)Galey (1960)
Fig. 1. Cross section for the D(, n)p reaction versus -ray ener