324 research outputs found
The Effects of Dark Matter Decay and Annihilation on the High-Redshift 21 cm Background
The radiation background produced by the 21 cm spin-flip transition of
neutral hydrogen at high redshifts can be a pristine probe of fundamental
physics and cosmology. At z~30-300, the intergalactic medium (IGM) is visible
in 21 cm absorption against the cosmic microwave background (CMB), with a
strength that depends on the thermal (and ionization) history of the IGM. Here
we examine the constraints this background can place on dark matter decay and
annihilation, which could heat and ionize the IGM through the production of
high-energy particles. Using a simple model for dark matter decay, we show
that, if the decay energy is immediately injected into the IGM, the 21 cm
background can detect energy injection rates >10^{-24} eV cm^{-3} sec^{-1}. If
all the dark matter is subject to decay, this allows us to constrain dark
matter lifetimes <10^{27} sec. Such energy injection rates are much smaller
than those typically probed by the CMB power spectra. The expected brightness
temperature fluctuations at z~50 are a fraction of a mK and can vary from the
standard calculation by up to an order of magnitude, although the difference
can be significantly smaller if some of the decay products free stream to lower
redshifts. For self-annihilating dark matter, the fluctuation amplitude can
differ by a factor <2 from the standard calculation at z~50. Note also that, in
contrast to the CMB, the 21 cm probe is sensitive to both the ionization
fraction and the IGM temperature, in principle allowing better constraints on
the decay process and heating history. We also show that strong IGM heating and
ionization can lead to an enhanced H_2 abundance, which may affect the earliest
generations of stars and galaxies.Comment: submitted to Phys Rev D, 14 pages, 8 figure
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter
Rate coefficients for rovibrational transitions in H_2 due to collisions with He
We present quantum mechanical and quasiclassical trajectory calculations of
cross sections for rovibrational transitions in ortho- and para-H_2 induced by
collisions with He atoms. Cross sections were obtained for kinetic energies
between 10^-4 and 3 eV, and the corresponding rate coefficients were calculated
for the temperature range 100<T<4000 K. Comparisons are made with previous
calculations.Comment: 21 pages, 2 figures, AAS, eps
Astrochemistry in the Early Universe: Collisional Rates for H on H2
We present preliminary results of a full quantum calculation of state to state cross sections for H on H2. These cross sections are calculated for v=0,4 j=0,15 for energies up to 3.0 eV. The cross sections are calculated on the BKMP2 potential surface (Boothroyd et al. 1996) with the ABC scattering code (Skouteris et al. 2000)
A multi-transition HCN and HCO+ study of 12 nearby active galaxies: AGN versus SB environments
Recent studies have indicated that the HCN-to-CO(J=1-0) and
HCO+-to-HCN(J=1-0) ratios are significantly different between galaxies with AGN
(active galactic nucleus) and SB (starburst) signatures. In order to study the
molecular gas properties in active galaxies and search for differences between
AGN and SB environments, we observed the HCN(J=1-0), (J=2-1), (J=3-2),
HCO+(J=1-0) and HCO+(J=3-2), emission with the IRAM 30m in the centre of 12
nearby active galaxies which either exhibit nuclear SB and/or AGN signatures.
Consistent with previous results, we find a significant difference of the
HCN(J=2-1)-to-HCN(J=1-0), HCN(J=3-2)-to-HCN(J=1-0), HCO+(J=3-2)-to-HCO+(J=3-2),
and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and
those with an additional or pure central SB: the HCN, HCO+ and HCO+-to-HCN
intensity ratios tend to be higher in the galaxies of our sample with a central
SB as opposed to the pure AGN cases which show rather low intensity ratios.
Based on an LVG analysis of these data, i.e., assuming purely collisional
excitation, the (average) molecular gas densities in the SB dominated sources
of our sample seem to be systematically higher than in the AGN sources. The LVG
analysis seems to further support systematically higher HCN and/or lower HCO+
abundances as well as similar or higher gas temperatures in AGN compared to the
SB sources of our sample. Also, we find that the HCN-to-CO ratios decrease with
increasing rotational number J for the AGN while they stay mostly constant for
the SB sources.Comment: accepted for publication in ApJ; 20 pages, 7 figures; in emulateApJ
forma
HNC, HCN and CN in Seyfert galaxies
Bright HNC 1--0 emission has been found towards several Seyfert galaxies.
This is unexpected since traditionally HNC is a tracer of cold (10 K) gas, and
the molecular gas of luminous galaxies like Seyferts is thought to have bulk
kinetic temperatures surpassing 50 K. In this work we aim to distinguish the
cause of the bright HNC and to model the physical conditions of the HNC and HCN
emitting gas. We have used SEST, JCMT and IRAM 30m telescopes to observe HNC
3-2 and HCN 3-2 line emission in a selection of 5 HNC-luminous Seyfert
galaxies. We estimate and discuss the excitation conditions of HCN and HNC in
NGC 1068, NGC 3079, NGC 2623 and NGC 7469, based on the observed 3-2/1-0 line
intensity ratios. We also observed CN 1-0 and 2-1 emission and discuss its role
in photon and X-ray dominated regions. HNC 3-2 was detected in 3 galaxies (NGC
3079, NGC 1068 and NGC 2623). HCN 3-2 was detected in NGC 3079, NGC 1068 and
NGC 1365. The HCN 3-2/1-0 ratio is lower than 0.3 only in NGC 3079, whereas the
HNC 3-2/1-0 ratio is larger than 0.3 only in NGC 2623. The HCN/HNC 1-0 and 3-2
line ratios are larger than unity in all the galaxies. The HCN/HNC 3-2 line
ratio is lower than unity only in NGC 2623, similar to Arp 220, Mrk 231 and NGC
4418. In three of the galaxies the HNC emissions emerge from gas of densities
n<10^5 cm^3, where the chemistry is dominated by ion-neutral reactions. In NGC
1068 the emission of HNC emerges from lower (<10^5 cm^3) density gas than HCN
(>10^5 cm^3). Instead, the emissions of HNC and HCN emerge from the same gas in
NGC 3079. The observed HCN/HNC and CN/HCN line ratios favor a PDR scenario,
rather than an XDR one. However, the N(HNC)/N(HCN) column density ratios
obtained for NGC 3079 can be found only in XDR environments.Comment: Accepted for publication in A&A. A selection of this paper will be
presented as a poster in the FIR workshop 2007, held at Bad Honnef, Germany.
High resolution figures in original paper. 16 pages, 8 figure
Tracing star formation in galaxies with molecular line and continuum observations
We report our recent progress on extragalactic spectroscopic and continuum
observations, including HCN(J=1-0), HCO(J=1-0), and CN(N=1-0) imaging
surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter
Array, high-J CO observations (J=3-2 observations using the Atacama
Submillimeter Telescope Experiment (ASTE) and J=2-1 observations with the
Submillimeter Array) of galaxies, and 1.1 mm continuum observations
of high-z violent starburst galaxies using the bolometer camera AzTEC mounted
on ASTE.Comment: 6 pages, 5 figures, To appear in proceedings of "Far-Infrared and
Submillimeter Emission of the Interstellar Medium", EAS Publication Series,
Bad Honnef, November 2007, Eds. C. Kramer, S. Aalto, R. Simon. See
http://www.nro.nao.ac.jp/~f0212kk/FIR07/kk-ver20.pdf for a version with high
resolution figure
When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars
Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared
emission bands that have been observed in a wide range of astrophysical
environments, where carbonaceous material is subjected to ultraviolet (UV)
radiation. Although PAHs are expected to form in carbon-rich AGB stars, they
have up to now only been observed in binary systems where a hot companion
provides a hard radiation field. In this letter, we present low-resolution
infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type
AGB stars observed with the infrared spectrograph aboard the Spitzer satellite.
The spectra of these four stars show the typical infrared features of PAH
molecules. We confirm the correlation between the temperature of the central
star and the centroid wavelength of the 7.9 {\mu}m feature, present in a wide
variety of stars spanning a temperature range from 3 000 to 12 000 K. Three of
four sources presented in this paper extend this relation towards lower
temperatures. We argue that the mixture of hydrocarbons we see in these S-stars
has a rich aliphatic component. The fourth star, BZ CMa, deviates from this
correlation. Based on the similarity with the evolved binary TU Tau, we predict
that BZ CMa has a hot companion as well.Comment: 5 pages, 2 figures, 2 table
HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium
We have detected new HD absorption systems at high redshifts, z_abs=2.626 and
z_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170,
respectively. Each of these systems consists of two subsystems. The HD column
densities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2)
and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208
and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20
for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for
three of these subsystems has been found to be considerably higher than its
values typical of clouds in our Galaxy. We discuss the problem of determining
the primordial deuterium abundance, which is most sensitive to the baryon
density of the Universe \Omega_{b}. Using a well-known model for the chemistry
of a molecular cloud, we have estimated the isotopic ratio
D/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density
\Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with
\Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmic
microwave background radiation anisotropy. However, in high-redshift clouds,
under conditions of low metallicity and low dust content, hydrogen may be
incompletely molecularized even in the case of self-shielding. In this
situation, the HD/2H_2 ratio may not correspond to the actual D/H isotopic
ratio. We have estimated the cloud molecularization dynamics and the influence
of cosmological evolutionary effects on it
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as
it initiates a network of chemical reactions that produce many interstellar
molecules. In dense clouds, the H3+ abundance is understood using a simple
chemical model, from which observations of H3+ yield valuable estimates of
cloud path length, density, and temperature. On the other hand, observations of
diffuse clouds have suggested that H3+ is considerably more abundant than
expected from the chemical models. However, diffuse cloud models have been
hampered by the uncertain values of three key parameters: the rate of H3+
destruction by electrons, the electron fraction, and the cosmic-ray ionisation
rate. Here we report a direct experimental measurement of the H3+ destruction
rate under nearly interstellar conditions. We also report the observation of
H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is
already known. Taken together, these results allow us to derive the value of
the third uncertain model parameter: we find that the cosmic-ray ionisation
rate in this sightline is forty times faster than previously assumed. If such a
high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy
between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres
- …