Arcetri Astrophysical Observatory

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Cosmic rays explain the origin of atomic hydrogen in dark clouds

The formation of molecular hydrogen occurs on dust grains in molecular clouds through the reaction between two hydrogen atoms. Because this is an exothermic process, H2 is then released into the gas phase. Depending on the position in the cloud (or on the amount of visual extinction measured inward from the cloud edge), two processes determine the destruction of H2 and the restoration of the atomic form: photodissociation that is due to interstellar UV photons, and dissociation due to cosmic rays. In the diffuse part of molecular clouds, UV photons regulate the abundance of atomic hydrogen by dissociating H2, while in the densest parts, interstellar UV photons are blocked by dust absorption as well as by H2 line absorption. In the deepest parts of the cloud, cosmic rays dominate the destruction of molecular hydrogen.

Small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, are a well-known characteristic of dark clouds.
A wealth of studies has been carried out to characterise the origin of the atomic hydrogen component in dense environments [1,2,3,4,5], but the rate of cosmic-ray dissociation was always assumed to be constant (i.e., independent of the position in the cloud) or was simply neglected.

It is crucial to accurately determine the abundance of atomic hydrogen since it is the most mobile reactive species on the surface of bare dust grains and icy mantles. As a result, the atomic hydrogen fraction determines the production of species such as formaldehyde (H2CO) and methanol (CH3OH) from the hydrogenation of CO and ammonia (NH3) from the hydrogenation of nitrogen.

A group of researchers - led by the Astrofit2 fellow Marco Padovani and including Daniele Galli of the Osservatorio Astrofisico di Arcetri - explored the role of cosmic rays in great detail, especially after the latest data release of the Voyager 1 spacecraft [6], which showed that the measured proton and electron fluxes are not able to explain the values of the cosmic-ray ionisation rate estimated in diffuse clouds [7].

Padovani et al. (2018) modelled the attenuation of the interstellar cosmic rays that enter a cloud and computed the dissociation rate of molecular hydrogen. They found that the cosmic-ray dissociation rate is entirely determined by secondary electrons produced in primary ionisation collisions. These secondary particles constitute the only source of atomic hydrogen at column densities above about 1021 cm-2.

From comparison with observations, they concluded that a relatively flat spectrum of interstellar cosmic-ray protons, such as suggested by the most recent Voyager 1 data, can only provide a lower bound for the observed atomic hydrogen fraction. An enhanced spectrum of low-energy protons is needed to explain most of the observations, as predicted in their previous works [8,9,10,11].

This paper shows that a careful description of H2 dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.


Figure: Atomic hydrogen fraction vs. the total column density of hydrogen (bottom scale) and visual extinction (top scale). Observations from [4] are shown as solid orange circles. Coloured stripes represent the models by Padovani et al. (2018) for the case of photodissociation only (purple), a Voyager-like cosmic-ray flux (model L, black) and an enhanced flux at low energies (model H, grey). Dashed lines refer to the average value of the total volume density of hydrogen (suggested by [4]).

[1] McCutcheon et al. (1978)
[2] van der Werf et al. (1988)
[3] Montgomery et al. (1995)
[4] Li & Goldsmith (2003)
[5] Goldsmith & Li (2005)
[6] Cummings et al. (2016)
[7] Indriolo et al. (2015)
[8] Padovani et al. (2009)
[9] Padovani et al. (2013)
[10] Ivlev et al. (2015)
[11] Padovani et al. (2018)

A Dynamical study of young open clusters

with the Gaia-ESO Survey


Stars do not form in isolation but in clusters composed of a few tens to several thousands of objects, which in most cases disperse in the Galactic field within 10-100 Myr. The physical mechanisms driving the clusters dissolution are not well understood and very debated.

In particular, two main scenarios have been proposed. According to the “residual gas expulsion model”, the cluster dispersion is driven by the feedback of high mass stars, which disperses the interstellar gas, that did not form stars. Other authors suggest that clusters evolution is driven by two-body interaction and the feedback is not relevant. An important parameter to distinguish between the two models is the virial ratio, which describes the dynamical status of the cluster, and -- under some assumptions -- can be derived by the velocity dispersion, the total cluster mass and its radius.

Luca Bravi, a PhD student of the University of Florence, and a group of researchers of the Arcetri Observatory (G. Sacco, S. Randich, E. Franciosini, E. Pancino, V. Roccatagliata, L. Magrini and L. Morbidelli) used the data of the Gaia-ESO Survey to determine for the first time the virial ratio of four open clusters in the critical age range between 20 and 50 Myr. As shown in Fig. 1, the excellent quality of the radial velocities allowed them to measure dispersions as small as 0.5 km/s, while the stellar parameters (see Fig. 2) have been used to derive the masses of the individual stars.

All the clusters studied are supervirial. This results is in agreement with the  “residual gas expulsion scenario” which suggests that the clusters are left in this state after the gas is swept out from the massive stars. The results of this work have been published in a paper on A&A (Bravi et al. , 2018).

Fig1 Luca PR

Fig. 1: Radial velocity distribution of the four open clusters studied by Bravi et al. (2018). The dashed lines represent the models used to fit the data.

Fig2 Luca PR

Fig. 2: HR diagrams of the four open clusters studied by Bravi et al. (2018). The stars are color-coded according to the probability to belong to clusters. Lines represent tracks and isochrones from the evolutionary models of Tognelli et al. (2011).

A protocluster of galaxies at z ~ 1.4 observed with LBT

Galaxy clusters provide an efficient tool for deriving cosmological parameters and for studying galaxy formation and evolution. A technique for detecting galaxy clusters at z > 1 has been to look at the immediate surroundings of high-redshift radio galaxies. An important aspect in the study of clusters is the demographics and distribution of the active galactic nuclei (AGN) within clusters and their evolution with redshift. The AGN population of a cluster has indeed important implications for the AGN fueling processes and how tightly black holes at the centers of cluster galaxies and galaxies coevolve.

In this scientific framework, a group of researchers -led by Viviana Casasola and including Laura Magrini and Simone Bianchi of the Arcetri Observatory- has recently published a paper on the spectroscopic study of 13 galaxies identified in the field of the protocluster associated with the radio galaxy 7C 1756+6520 at z = 1.4156 (Casasola et al. 2018). This work, stimulated by previous results obtained for this protocluster (Magrini et al. 2012, Casasola et al. 2013), has been performed on rest-frame optical spectra taken with the Large Binocular Telescope (LBT). The adopted spectral coverage allowed to observe emission lines such as Hα, Hβ, [O III]5007 Å, and [N II]6583 Å at the redshift of the central radio galaxy.

With these LBT observations, Casasola et al. (2018)derived the redshift by detecting emission lines that have never detected before for these galaxies. They also identified a new protocluster member and eight new possible protocluster members. These galaxies are now identified with the name CMC [Casasola, Magrini, Combes, from the names of the first three authors of the paper] followed by a number. Figure 1 shows the spatial distribution of the sources Casasola et al. (2018) detected in the field of the protocluster 7C 1756+6520 (left panel) and of all the galaxies spectroscopically confirmed members of the galaxy overdensity (right panel).  


Figure 1:Left panel: Galaxies detected by Casasola et al. (2018) in the field of the overdensity around the radio galaxy 7C 1756+6520 plotted on the B-band image (NOAO). The five colors and symbols indicate different ranges of redshift. The central radio galaxy 7C 1756+6520 is the big yellow star in the center. Right panel: All protocluster and possible protocluster members with spectroscopically confirmed redshift from this work and the literature. The circle indicates a distance of 2 Mpc from the central radio galaxy.

The stacked spectrum of the galaxies in which Casasola et al. (2018) detected the [O III]5007 Å emission line, shown in Figure 2, revealed the presence of the second line of the [O III] doublet at 4959 Å and of Hβ. Additionally, the ratio between the two lines of the [O III] doublet is consistent with the theoretical value of ~3, confirming that these galaxies belong to the protocluster.


Figure 2: 1D stacked rest-frame spectrum in the J-LBT wavelength range of the possible protocluster members where Casasola et al. (2018) detected the [O III]5007 Å line.

A peculiar property of this protocluster is its AGN fraction, which at 23% is higher than what typically characterizes low-, moderate- and high-redshift clusters. The high AGN fraction and distribution of AGN within the protocluster seem to be broadly consistent with predictions of some theoretical models on the AGN feedback, based on galaxy interactions and ram pressure.

For one protocluster AGN (AGN.1317), Casasola et al. (2018) also confirmed, for the first time through two “Baldwin, Phillips & Terlevich” (BPT) diagrams, that it hosts an AGN. This finding is illustrated in Figure 3.


Figure 3:The [N II]/Hα vs. [O III]/Hβ (left panel) and [S II]/Hα vs. [O III]/Hβ  (right panel) BPT diagrams showing the location of AGN.1317 (yellow triangle).

Observations of AGN in this protocluster and in other distant clusters will help clarifying whether the resulting high fraction of AGN is unusual or typical for such structures at high redshift.



Casasola et al. (2018), A&A, in press

Casasola et al. (2013), A&A, 558, 60

Magrini et al. (2012), MNRAS, 426, 1195

Gravity versus feedback: shaping the gas distribution in M33

The Arcetri extragalactic group has always been active in investigating Local Group closest spirals, M31 and M33, performing measurements of dark matter halo properties, imaging and modeling their stellar and interstellar medium (ISM) constituents. Dark matter masses, together with proper motion data analysis, suggest that M33 is an undisturbed disk on its first approach to M31 with no sign of recent merging of satellite galaxies. Hence, this blue spiral galaxy is the closest laboratory to study the driving physical processes for spiral structure, gas fragmentation and star formation in isolated disks. A fundamental open question is related, for example, to the low star formation efficiency. This can be the result of localized small collapsing regions inside large clouds dominated by turbulence. Another possibility is that cloud complexes are collapsing on a much larger scale but feedback from newborn massive stars quenches star formation.

To shed light on the spatial scale of self-gravitating structures, a group of scientists led by Edvige Corbelli has recently investigated the shape of the probability distribution functions (PDFs) of the atomic, molecular and total gas surface density (Corbelli, Elmegreen, Braine, Thilker 2018) throughout the whole disk of M33. Numerical simulations have shown that compressible turbulence, driven by feedbacks, shapes the gas PDF as a log-normal function but, if self-gravity becomes dominant above a certain density in a cloud, the PDF develops a power-law tail. Hence, the lower limit to the power-law part of the PDF gives the average column density above which self-gravity becomes the main driving force of cloud evolution. The state-of-the-art spatial resolution and frequency coverage of recent ISM surveys have allowed Corbelli et al. (2018) to determine for the first time the average transition column density for several regions (defined in Figure 1) throughout a star forming disk, combining molecular and atomic data.  The analysis has been performed also for separate sets of giant molecular clouds (GMCs), each representing clouds at a particular stage during their evolutionary sequence, as defined by Corbelli et al. (2017).

Edvige Figure1
Figure 1: Velocity dispersion map of the atomic hydrogen 21cm line emission in M33 at 10 arcsec resolution (left panel). The  black contours underline the location of the bright HI filaments or arms. These contours are overlaid also on the surface density map in the right panel where the magenta boxes indicate regions selected for this study: the central region (CR), the northern arm (NA), the southern arm (SA), the outermost filament to the east (EF). Figure from Corbelli et al. (2018).

The average velocity dispersion of the atomic gas in M33 (Figure 1) is radially constant with values on the order of 13 km/s suggesting a ubiquitous presence of feedback driven turbulence. The high pressure and rate of star formation in the central region shapes the PDFs as log-normal functions for all gas components. GMCs prior and during star formation in the central region and in the northern arm are dominated by turbulence and intense feedback at all column densities that are spatially resolved. In the southern arm and especially in the outermost filament rich in molecules, large departures of the molecular gas PDF from a log-normal shape are found, with power-laws detected for column densities  above 1021 H atoms/cm2 (see Figure 2). This, combined with cloud maps, suggests a stratification of density within molecular cloud complexes, compatible with the dominance of self-gravity at all evolutionary stages in these regions. Moving radially outwards in the disk, gravity takes over turbulence on larger scales, likely because turbulence becomes subsonic as suggested by the decreasing PDF width and by cloud models discussed in detail in the paper.

Edvige Figure2
Figure 2: PDFs of the molecular gas column density in units of H atoms/cm2 for the total molecular gas (black heavy curves), and for the gas in three GMC types: non-star forming (red lines), with embedded star formation (blue lines), and with exposed star formation (green lines).The PDF for diffuse molecular gas (meaning low-mass clouds or truly diffuse) is marked in magenta (see Corbelli et al. 2018 for more details). All PDFs have been normalized to the number of pixels at the peak. The panels correspond to the all-disk and to different regions in the galaxy. Figure adapted from Corbelli et al (2018).

The role of feedback and gravity in driving ISM structures in M33 has been also investigated numerically using smoothed particle hydrodynamics simulations by Dobbs, Petit, Corbelli and Pringle (2018). This study has shown for the first time that gravitational instabilities in the stars and gas are able to drive the formation of filamentary spiral arms in the ISM of isolated disks. However, to reproduce in detail the flocculent spiral pattern and the prominent holes observed in the atomic gas of M33, a sufficiently high stellar feedback efficiency is needed as can be seen in Figure 3. Data analysis and models of M33, the closest undisturbed spiral galaxy, clearly show the delicate balance between gravity and feedback at all scales during galaxy evolution.

Edvige Figure3
Figure 3: Atomic gas surface density according to best fitting simulated models for different levels of feedbacks compared to the observed M33 atomic gas distribution by Corbelli et al. 2014 (lower right). Figure from Dobbs et al (2018).

 Corbelli, E., Thilker, D.,  Zibetti, S., Giovanardi, C., and Salucci, P., 2014, A&A  572, 23
 Corbelli, E., Braine, J., Bandiera et al. 2017, A&A 601, 146
 Corbelli, E., Elmegreen, B. G., Braine, J., and Thilker, D., 2018, A&A in press 
 Dobbs, C. L., Petit, A. R., Corbelli, E., and Pringle, J. E. 2018, MNRAS, 478, 3793  

Edited by E. Corbelli and A. Gallazzi

Phosphorus chemistry in star forming regions:  

understanding the formation of the molecule of PN

Together with Carbon, Oxygen, Hydrogen and Nitrogen, Phosphorus is one of the key element for Life as we know it on Earth. The tight connection of Life and Phosphorus is shown by the presence of this element in several biologically relevant molecules: it plays a central role in nucleic acids (DNA and RNA), phospholipids (the “skin" of all cellular membranes) and the adenosine triphosphate (ATP), from which all forms of life assume energy. [1]



All the Phosphorus in the Universe is created through nuclear reactions in high mass stars (stars with M>8 M⦿) and it is injected to the interstellar medium (ISM) through supernovae explosions [2], butthe chemistry of this element during the process of star formation is still poorly known.
The only two P-bearing molecules detected in dense star forming clouds are the two simple molecules of PN and PO. Until recent years the few number of detections were not enough to understand which are the physical conditions that favour the formation of P-bearing molecules. In particular, PN is a crucial species to understand the chemistry of interstellar P, as it has been proposed as precursor of other P-bearing species like PO, HNNP, HNPN, and HPNN [3,4]. Moreover, PN-based derivatives have been proposed as very plausible prebiotic agents in the early Earth [5].
For these reasons, the molecule of PN has been the central point of the study led at the Arcetri Observatory by a group of researchers, including Chiara Mininni (PhD Student), Francesco Fontani, Victor Manuel Rivilla and Maite Beltrán. In this study the molecule of PN has been observed towards nine high-mass star forming regions in different evolutionary stages, in its rotational transitions at 1 mm and 2.1 mm. The data were obtained using the IRAM-30m, located at Pico Veleta in Spain, and were integrated with the observation of PN line at 3.2 mm presented in [6]. PN was detected, at least in one transition, in all the nine sources, regardless of the different evolutionary stages.
The detection of more than one spectral line of PN has allowed the researchers to calculate the abundances of this molecule in the sources, using the method of Rotational Diagrams. These abundances has been compared to those of other well known molecules, used as tracers of different physical condition and chemical pathways:
  • SiO and SO: they are present in the nuclei of dust grains and their abundances in gas phase are enhanced in regions of shocks.
  • CH3OH: it forms on the surface of dust grains and it is mainly released in gas phase due to thermal heating.
  • N2H+: it forms via gas-phase chemical reactions.
The comparisons of the abundances and of the line widths of the lines seem to exclude any correlation between PN and CH3OH, while there is a faint but statistically significant positive trend between the abundances of PN and those of N2H+, SiO and SO (see Figure 1)
Figure 1: In the panel on the left is shown the plot of the abundances of PN against the abundances of the molecule of SO, while in the right panel the PN abundances is plotted against those of CH3OH. The red line is the best linear fit; the angular coefficient of it is reported in the upper left corner of each panel. Different color refers to evolutionary stages (High Mass Starless Core, High Mass Protostellar Object and Ultra Compact HII region)
The main result of the analysis is that in six out of nine sources line profiles of PN are very well correlated with those of the two shock tracers SiO and SO (see Figure 2).
This, together with the positive trend shown by the abundances, seems to point out that in 2/3 of the sample the most important release mechanism of PN is sputtering of dust grains in shocked regions, in good agreement with recent results in Galactic Center clouds [7].
Nevertheless, this can not be the only mechanism, since the line profiles of the three remaining sources do not show high-velocity wings (associated with shocked material), but narrow line widths (these sources has been labeled as Narrow (N), while the previous as Broad (B) in Figure 1 and 2).
Figure 2: For each of the nine sources we present the overplot of the PN (3-2) line (in red) and the shock tracer SiO (2-1) line (in black). The lines of the molecule of PN are multiplied for an appropriate factor (reported in red in the upper left corner of each panel), in order to be more visible. The Broad sources (B) shows good agreement in the lines profile, while this is not true for Narrow sources (N)
This confirms the results of Fontani et al. (2016), who found line widths for the line at 3.2 mm lower than 5 km/s in some sources, and reinforces the conclusion that the origin of PN is not to be considered unique, since it could form in both shocked and quiescent gas.
Paper:“On the origin of phosphorus nitride in star-forming regions”, accepted for publication in Monthly Notices of the Royal Astronomical Society Letters
Authors: C. Mininni, F. Fontani, V. M. Rivilla, M. T. Beltrán, P. Caselli & A. Vasyunin
[1] Pasek and Lauretta, Astrobiology 5, 515-535 (2005)
[2] Koo et al., Science 342, pp 1346-1348 (2013)
[3] Rivilla et al., ApJ 826, 161 (2016)
[4] Bhasi et al., JTCC 16, 1750075 (2017)
[5] Karki et al., Life 7, 32 (2017)
[6] Fontani et al., ApJ 822, L30 (2016)
[7] Rivilla et al., MNRAS 475, L30-L34 (2018)