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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).

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  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.

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 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.

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 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.

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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