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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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