Detection of CO+ in the nucleus of M82

We present the detection of the reactive ion CO+ towards the prototypical starburst galaxy M82. This is the first secure detection of this short-lived ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+] ratio had only been previously measured towards the atomic peak in the reflection nebula NGC7023. This detection corroborates that the molecular gas reservoir in the M82 disk is heavily affected by the UV radiation from the recently formed stars. Comparing the column densities measured in M82 with those found in prototypical Galactic photon-dominated regions (PDRs), we need \~20 clouds along the line of sight to explain our observations. We have completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR chemical model successfully explains the [CO+]/[HCO+] ratios measured in the M~82 nucleus but fails by one order of magnitude to explain the large measured CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to reconcile the chemical model and the observations.Comment: 12 pages, 2 figure

Molecular line probes of activity in galaxies

The use of specific tracers of the dense molecular gas phase can help to explore the feedback of activity on the interstellar medium (ISM) in galaxies. This information is a key to any quantitative assessment of the efficiency of the star formation process in galaxies. We present the results of a survey devoted to probe the feedback of activity through the study of the excitation and chemistry of the dense molecular gas in a sample of local universe starbursts and active galactic nuclei (AGNs). Our sample includes also 17 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007) we find the first clear observational evidence that the star formation efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting evidence of overabundant HCN in active environments would even reinforce the reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt law around L_FIR=10^11 L_sun. This result has major implications for the use of HCN as a tracer of the dense gas in local and high-redshift luminous infrared galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07 (FIR 2007

Quantitative Analysis of the Effective Functional Structure in Yeast Glycolysis

Yeast glycolysis is considered the prototype of dissipative biochemical oscillators. In cellular conditions, under sinusoidal source of glucose, the activity of glycolytic enzymes can display either periodic, quasiperiodic or chaotic behavior. In order to quantify the functional connectivity for the glycolytic enzymes in dissipative conditions we have analyzed different catalytic patterns using the non-linear statistical tool of Transfer Entropy. The data were obtained by means of a yeast glycolytic model formed by three delay differential equations where the enzymatic speed functions of the irreversible stages have been explicitly considered. These enzymatic activity functions were previously modeled and tested experimentally by other different groups. In agreement with experimental conditions, the studied time series corresponded to a quasi-periodic route to chaos. The results of the analysis are three-fold: first, in addition to the classical topological structure characterized by the specific location of enzymes, substrates, products and feedback regulatory metabolites, an effective functional structure emerges in the modeled glycolytic system, which is dynamical and characterized by notable variations of the functional interactions. Second, the dynamical structure exhibits a metabolic invariant which constrains the functional attributes of the enzymes. Finally, in accordance with the classical biochemical studies, our numerical analysis reveals in a quantitative manner that the enzyme phosphofructokinase is the key-core of the metabolic system, behaving for all conditions as the main source of the effective causal flows in yeast glycolysis.Comment: Biologically improve

ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio

We have observed the S(0), S(1), S(2), S(3), S(4) and S(5) rotational lines of molecular hydrogen (H2) towards the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300 - 700 K. However, the data cannot be fitted by an ortho- to para- (OTP) ratio of 3. An OTP ratio in the range ~1.5 - 2 is necessary to explain our observations. This is the first detection of a non-equilibrium OTP ratio measured from the H2 pure-rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.Comment: 4 pages, 3 figure

First evidence for dusty disks around Herbig Be stars

We have carried out a high-sensitivity search for circumstellar disks around Herbig Be stars in the continuum at 1.4mm and 2.7mm using the IRAM interferometer at the Plateau de Bure (PdBI) . In this letter, we report data on three well studied B0 stars, MWC 1080, MWC 137 and R Mon. The two latter have also been observed in the continuum at 0.7 cm and 1.3 cm using the NRAO Very Large Array (VLA) . We report the detection of circumstellar disks around MWC 1080 and R Mon with masses of Md ~ 0.003 and 0.01 Msun, respectively, while for MWC 137 we estimate a disk mass upper limit of 0.007 Msun. Our results show that the ratio Md/M* is at least an order of magnitude lower in Herbig Be stars than in Herbig Ae and T Tauri stars.Comment: 5 pages (including figures

Cluster Origin of Triple Star HD 188753 and its Planet

The recent discovery by M. Konacki of a ``hot Jupiter'' in the hierarchical triple star system HD 188753 challenges established theories of giant-planet formation. If the orbital geometry of the triple has not changed since the birth of the planet, then a disk around the planetary host star would probably have been too compact and too hot for a Jovian planet to form by the core-accretion model or gravitational collapse. This paradox is resolved if the star was initially either single or had a much more distant companion. It is suggested here that a close multi-star dynamical encounter transformed this initial state into the observed triple, an idea that follows naturally if HD 188753 formed in a moderately dense stellar system--perhaps an open cluster--that has since dissolved. Three distinct types of encounters are investigated. The most robust scenario involves an initially single planetary host star that changes places with the outlying member of a pre-existing hierarchical triple.Comment: Accepted by ApJL; minor changes from origina

Insights into the Carbon chemistry of Mon R2

Aiming to learn about the chemistry of the dense PDR around the ultracompact (UC) HII region in Mon R2, we have observed a series of mm-wavelength transitions of C3H2 and C2H. In addition, we have traced the distribution of other molecules, such as H13CO+, SiO, HCO, and HC3N. These data, together with the reactive ions recently detected, have been considered to determine the physical conditions and to model the PDR chemistry. We then identified two kind of molecules. The first group, formed by the reactive ions (CO+, HOC+) and small hydrocarbons (C2H, C3H2), traces the surface layers of the PDR and is presumably exposed to a high UV field (hence we called it as "high UV", or HUV). HUV species is expected to dominate for visual absorptions 2 < Av < 5 mag. A second group (less exposed to the UV field, and hence called "low UV", or LUV) includes HCO and SiO, and is mainly present at the edges of the PDR (Av > 5 mag). While the abundances of the HUV molecules can be explained by gas phase models, this is not the case for the studied LUV ones. Although some efficient gas-phase reactions might be lacking, grain chemistry sounds like a probable mechanism able to explain the observed enhancement of HCO and SiO. Within this scenario, the interaction of UV photons with grains produces an important effect on the molecular gas chemistry and constitutes the first evidence of an ionization front created by the UC HII region carving its host molecular cloud. The physical conditions and kinematics of the gas layer which surrounds the UC HII region were derived from the HUV molecules. Molecular hydrogen densities > 4 10^6 cm^(-3) are required to reproduce the observations. Such high densities suggest that the HII region could be pressure-confined by the surrounding high density molecular gas.Comment: 32 pages, 8 figures. Accepted by Astrophysical Journa