Arcetri Astrophysical Observatory

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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)
chiara1
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).
chiara2
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
http://adsabs.harvard.edu/abs/2018arXiv180200623M
[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)
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Dynamical decomposition of galaxies reveals the unquiet life of disks

The motions of stars in galaxies are the result of their formation processes, of the internal dynamical evolution of galaxies, and of the episodes of accretion and merger that galaxies may experience along their life. Hence analyzing how stars at different locations and with different ages or chemical compositions orbit within galaxies provide invaluable information to understand how galaxies formed and evolve. This task, however, is extremely challenging and requires advanced instrumentation and observational techniques to map the kinematics and the properties of stars, and utterly complex analysis methods to model the physical state and evolution of the systems.

A group of scientists, led by Ling Zhu of the Max Planck Institute for Astronomy in Heidelberg and including Stefano Zibetti from the INAF-Arcetri Astrophysical Observatory, has made a big step in this direction with the results presented in a paper published on January 1st, 2018 in Nature Astronomy, "The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey" (also available here). They have analyzed the statistical distribution of the orbits of stars for the first time in a representative sample of present-day galaxies. The team has exploited the wealth of exquisite Integral-Field Spectroscopic data (i.e. spatially resolved spectroscopy) collected by the CALIFA (Calar Alto Legacy Integral Field Area) survey to obtain detailed kinematic maps of 300 galaxies. The observations were modeled using the Schwarzschild orbit-superposition method to decompose the stellar population of each galaxy into three different families of orbits: "cold" orbits, representing the ordered circular motions of stars in galactic disks; "hot" orbits, characteristic of chaotic motions occurring in spheroidal components, such as the bulges and the haloes; "warm" orbits, with a significant rotational component, yet with non-negligible random motions, typical of thick disks and "pseudo-bulges" (see Figure 1).

orbit add crop
Figure 1: Models of stellar orbits (left) are matched to the observed images and maps of stellar velocity and velocity dispersion measured from the CALIFA spectra (right; example for the galaxy NGC001 in the sample). Credit: Instituto de Astrofisica de Andalucia (IAA-CSIC). Adapted from figure 1 of Zhu et al, 2018.

The statistical distribution of stars in these three orbital families, as a function of galaxy properties, is a key benchmark and will be of fundamental importance to constrain galaxy evolution models and simulations of the Universe (see Figure 2). Interestingly, this study finds that no galaxy is fully dominated by ordered circular motions. Even those galaxies that appear structurally dominated by a thin disk, actually have a majority of stars in warm orbits and a fraction on hot ones. This supports the idea that, although stars are born mainly in thin purely rotating disks, the so-called dynamical "secular evolution" driven by instabilities (spiral arms, bars) and minor interactions with external galaxies plays a major role in reshaping the orbital distribution of stars and in "warming up" the galactic disks.

Zhu Fig3Figure 2: Average fractional contribution of each of the three orbital components, plus counter-rotating (CR) orbits, as a function of the galaxy stellar mass. Source: figure 3 of Zhu et al, 2018.

For furher info, see "Quando le stelle perdono la bussola" on Media INAF.


Edited by S. Zibetti and A. Gallazzi

 

CO excitation in the Seyfert galaxy NGC 34: stars, shock or AGN driven?

In the last decades, intensive observational and theoretical investigations have demonstrated how the Star Formation (SF) of galaxies and the Active Galactic Nuclei (AGN) phenomena are deeply connected. There are many observational pieces of evidence that support this connection, such as the tight relations between the super massive black hole (BH) mass and the host galaxy properties or the similar shape of the SF and the BH accretion density as a function of the cosmic.

In this context, Matilde Mingozzi – Ph.D. student at the University of Bologna and at the Arcetri Astrophysical Observatory – and her collaborators (including Giovanni Cresci from the Arcetri Astrophysical Observatory) have studied the local galaxy NGC 34, where the two phenomena co-exist. The main aim of their work is assessing whether and to what extent the molecular gas emission, traced mainly by carbon monoxide (CO), is influenced by the radiation produced by the accretion onto the BH. In fact, molecular gas is a key component of the interstellar medium (ISM), as it is the fuel of SF and possibly of AGN accretion. In particular, the so-called CO Spectral Line Energy Distribution (CO SLED) – i.e. the CO line luminosity as a function of the CO upper rotational level – is a fundamental tool to constrain the physical properties of the molecular gas, such as density, temperature and the main source that causes the emission.

In their work, they analyse the X-ray and CO emission, using mainly archival data from XMM-Newton, NuSTAR, ALMA, and Herschel. On the one hand, the X-ray data allow to properly include the effect of AGN radiation in the modelling of the CO SLED, on the other hand, the CO emission, traced by ALMA in the central region of NGC 34 at high spatial resolution, is crucial to spatially constrain the region where the contribution of the AGN actually dominates. In order to fit the shape of the CO SLED, they make use of grids of models of Photo-Dissociation Regions (PDRs) and X-ray Dominated Regions (XDRs) – i.e. regions whose physics and chemistry are dominated by far ultraviolet radiation due to young stars and X-ray radiation coming from the AGN, respectively. In addition, they take into account also a grid of shock models, since shocks, originated from the supersonic injection of mass into the ISM, can also compress and heat the gas.

Matilde picture1
Figure 1: Fiducial model (PDR + XDR) over-plotted on the observed data. The light-blue dashed line and the red dotted line represent the low-density PDR and the high-density XDR, respectively. The black solid line indicates the sum of the two components and the shaded areas indicate the ±1σ uncertainty range on the normalization of each component. Taken from Mingozzi et al (2017).

In a recent paper, “CO excitation in the Seyfert galaxy NGC 34: stars, shock or AGN driven?” led by M. Minghozzi and accepted in MNRAS, the authors conclude that the AGN contribution is significant in heating the molecular gas in NGC 34 (Fig. 1 shows the best-fit of the CO SLED, composed by PDR and XDR components). Their results shed light on the great potential of combining self-consistent multi-band and multi-resolution data in order to assess the importance of AGN and SF activity, and mechanical heating produced by shocks for the physics of molecular gas.


Edited by M. Mingozzi and A. Gallazzi

Phosphorus-bearing molecules in the Galactic Center

Phosphorus (P) is one of the essential elements for life due to its central role in bio-chemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are present in star-forming regions, although their formation routes remain poorly understood. A group of researchers of the Arcetri Observatory, led by Victor Rivilla and including Francesco Fontani and Maria Teresa Beltran, has reported observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterizedby different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detectionscorrespond to clouds exposed to  intense UV/X-rays/cosmic-ray radiation. PO is detected only towards the cloud G+0.693-0.03, with a PO/PN abundance ratio of  1.5. They conclude that P-bearing molecules likely form in shocked gas as a result of dustgrain sputtering, while are  destroyed by intense UV/X-ray/cosmic ray radiation.

Phosphorus (P) is essential for life because it plays a centralrole in the formation ofmacromolecules such as phospholipids (the structural components of cellular membranes)and the deoxyribonucleic acid (DNA, Macia et al. 1997). For decades PN  remained as the only P-bearingspecies observed in these regions (Turner & Bally 1987; Ziurys1987; Yamaguchi et al. 2011Fontani et al. 2016), whilePO has been discovered just recently in the surroundingsof both high- and low-mass protostars  (with PO/PN abundanceratios of 1-3; Rivilla et al. 2016; Lefloch et al. 2016).

 Arcetri-highlight-fig1.jpg

  Figure 1: Sample of Galactic Center clouds we have observed, overplotted on an Spitzer-IRAC 4 image.

The formation of P-bearing molecules is still poorly understood. Three routes have been proposed: (i) shock-induced desorptionof P-bearing species (e.g. PH3) from dust grainsand subsequent gasphase formation (Aota & Aikawa 2012;Lefloch  et al. 2016); (ii) high-temperature gas-phase chemistryafter the thermal desorption of PH3 from ices (Charnley& Millar  1994); and (iii) gas-phase formation of PN andPO during the cold collapse phase andsubsequent thermaldesorption (at  temperatures 35 K) by protostellar heating (Rivilla et al. 2016). Due to the limited number of observationsavailable, and the  limited range of physical conditionsof the observed regions with detected P-bearing molecules,the formation routes for PN  and PO are strongly debated.

 Arcetri-highlight-fig3.jpg


 Figure 2:PN (2-1) and 29SiO (2-1) lines measured towards the Galactic Center clouds. The Local Thermodynamic Equilibrium best fits are shown with red lines. The PN molecule is only detected towards the sources dominated by shocks.

Victor Rivilla and collaborators have presented new observations of PN and POtowards seven regions spread across the  Central MolecularZone (CMZ) in the Galactic Center (GC) (see Figure 1). These sourcesare excellent laboratories to test the  chemistry of P-bearingmolecules since they show different physical properties (highkinetic temperatures, low dust  temperatures and moderatedensities) and chemistries dominated by either UV photons,cosmic-rays (CR), X-rays or shock  waves. The selected sample includes two different types of sources:(i) Shock-dominated regions; and (ii) Radiation dominated regions.

They have carried out observations at 3mm and 2mm using the radiotelescope IRAM 30m located at Pico Veleta (Granada,  Spain). PN is detected towards five of the seven sources (see Figure 2). PO is detected only towardsone of the sources,  G+0.693-0.03 (see Figure 3), which is thought to be therichest source of O-bearing molecules in the Galactic Center.The  derived PO/PN abundance ratio is 1.5, similar to valuespreviously found in star-forming regions.

Arcetri-highlight-fig2.jpg

 Figure 3: PO detection towards G+0.693-0.03 (lower panel) compared with the detection towards the hot molecular core W51  e1/e2from Rivilla et al. (2016) (upper panel). The PO quadruplet is shown with vertical blue lines. Other molecular  species are labeled in theupper panel. TheLocal Thermodynamic Equilibrium synthetic spectrum of PO in both sources is  shown with red lines.

The regions whereP-bearing species have been detected are clouds thought tobe affected by shock waves, and rich in the  well-knownshock tracer 29SiO (see Figure 4). The two sources where no P-bearingmolecules were detected are regions  exposed to intense radiation,and exhibit lower abundances of 29SiO. Wethus conclude that P-bearing species are formed in  thegas phase after the shock-induced sputtering of the grainmantles,and that they are efficiently destroyed by thehigh  cosmic-rays/X-rays/UV-photon radiation expected inthe Galactic Center.

Arcetri-highlight-fig4.jpg

Figure 4:Column density ratios of PN and 29SiO with respectto C34S. The different type of sources are Shock-dominated GCclouds (red dots) and Radiation-dominated regions (greenstars). The L1157-B1 shock(magenta open star) and the L1544 pre-stellar core (opendiamond) havealso been added. Arrows indicate 3 upper limits.


More info:

Contact: Víctor M. Rivilla, https://www.arcetri.astro.it/~rivilla/

Paper:Phosphorous-bearing molecules in the Galactic Center”, accepted for publication in Monthly Notices of the Royal Astronomical Society Letters; https://arxiv.org/abs/1712.07006