Massive Star Formation with New Generation Interferometers

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

Over the last 25 years, large improvements in the detection capabilities of current telescopes in the infrared (IR) and radio parts of the spectrum of light, have allowed us to carry out systematic studies of the population of stars present in clusters deeply embedded in molecular dark clouds across our Galaxy. These clusters, which remain hidden behind large amounts of interstellar molecular gas and dust, cannot be accessed by telescopes operating at optical wavelengths because the radiation emitted by young stars is absorbed by interstellar cold dust, which re-emits it afterwards at infrared and radio wavelengths. These IR and radio studies have revealed that most stars in our Galaxy form in clusters, and that these clusters typically show a wide distribution of masses that range from brown dwarfs, with 13-80 times the mass of Jupiter, to the most massive stars, with over a hundred times the mass of the Sun (or Mo). This mass distribution is called the Initial Mass Function (IMF) and peaks broadly between 0.1-0.5 Mo indicating that most stars forming in our Galaxy are low-mass stars. Massive stars - stars with masses larger than 8 Mo - are therefore rare. However, they dominate the energetics and feedback of galaxies, since they inject vast amounts of energy in the form of winds, strong ultraviolet (UV) radiation and supernovae explosions. Despite their short life times, massive stars dramatically modify the dynamical, thermal and chemical properties of the interstellar medium (ISM), shaping the structure of galaxies and largely impacting on galaxy evolution. Massive stars are also a key source of heavy elements, enriching the ISM for future generations of stars and for their associated solar systems. Understanding the physical processes that lead to the formation of massive stars is therefore crucial to establish their role in galaxy formation and evolution, and in the origin of life. However, in contrast with low-mass stars whose formation mechanisms are relatively well-known, the processes that lead to massive star birth still remain unclear. Observational studies of massive star forming regions are indeed challenging in modern astronomy because these regions are located at distances very far from us (further away than 1 kpc or 3000 light years), they show a complex morphology and structure with a high level of confusion, and they remain hidden behind large amounts of molecular gas and dust. The unprecedented capabilities of the new generation synthesis imaging radio telescopes such as the Submillimeter Array (SMA) on Mauna Kea (Hawaii), the Northern Extended Millimeter Array (NOEMA) located in the french Alps, and the Atacama Large Millimeter Array (ALMA) located in the Atacama desert in Chile, will represent a breakthrough in radio astronomy since they will allow us to image massive star forming regions with much higher detail (angular resolution), sensitivity (to discover new and fainter features), and at wavelength ranges never explored before. Molecules, which are very abundant in these regions, will be used as probes because they are very sensitive to the different physical processes (UV photon illumination or shock waves) taking place in the ISM. In this proposal, I will therefore combine the detailed information provided by the observations with these telescopes, with theoretical models of the chemistry of the ISM to understand the physical processes involved in the formation of the most massive stars in our Galaxy.

Publications

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Barnes A (2016) Widespread deuteration across the IRDC G035.39-00.33 in Monthly Notices of the Royal Astronomical Society

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Fuller G. A. (2016) The Science Case for ALMA Band 2 and Band 2+3 in arXiv e-prints

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Gómez-Ruiz A (2016) Diagnosing shock temperature with NH 3 and H 2 O profiles in Monthly Notices of the Royal Astronomical Society

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Henshaw J (2016) Investigating the structure and fragmentation of a highly filamentary IRDC in Monthly Notices of the Royal Astronomical Society

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Henshaw J (2017) Unveiling the early-stage anatomy of a protocluster hub with ALMA in Monthly Notices of the Royal Astronomical Society: Letters

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Holdship J (2016) H 2 S in the L1157-B1 bow shock in Monthly Notices of the Royal Astronomical Society

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Jiménez-Serra I (2016) The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core. in The astrophysical journal. Letters

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Vasyunin A (2017) Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach in The Astrophysical Journal

Related Projects

Project Reference Relationship Related To Start End Award Value
ST/L004801/1 01/02/2015 14/06/2016 £451,297
ST/L004801/2 Transfer ST/L004801/1 15/06/2016 31/07/2018 £325,705
 
Description Massive stars - stars with masses larger than 8 Mo - are known to dominate the energetics and feedback of galaxies since they inject vast amounts of energy in the form of jets/outflows, strong ultraviolet (UV) radiation and supernovae explosions throughout their lives. Despite their importance, the processes that lead to the formation of massive stars remain poorly known. It is currently believed that massive stars form in massive, filamentary and dark molecular clouds (known as Infrared-Dark Clouds or IRDCs).

The key findings of this period are summarised below:

1) IRDCs show fingerprints of their formation process via cloud-cloud collisions in the form of extended emission from molecules such as silicon monoxide (SiO) and methanol (CH3OH; Cosentino, Jimenez-Serra et al., paper in preparation), and the emission from mid-J rotational lines of carbon monoxide (CO; see Pon et al. 2016a, 2016b).

2) Massive Infrared-Dark Clouds (or IRDCs) show molecular species typical of very cold chemistry in the interstellar medium (e.g. deuterated species such as N2D+), which imply that these clouds represent the initial conditions of massive star and star cluster formation. The derived lifetime for these clouds is 3 Myr, i.e. 8 times longer than the free-fall time-scales. This implies that magnetic fields may play an important role in the formation and evolution of these clouds (Barnes et al. 2016; Zahorecz, Jimenez-Serra et al. 2017, submitted to A&A).

3) When observed at higher angular resolution with interferometers such as NOEMA and ALMA massive clumps located at the centre of filament-hub systems split up into multiple intra-hub filaments, each of which retains its integrity as an independent structure and possesses its own embedded core population. (Henshaw et al. 2016; McGuire et al. 2016; Henshaw, Jimenez-Serra et al. 2017).

4) The chemistry of isotopologues of nitrogen in low-density IRDCs (and in particular, the 15N fractionation) resembles that found in low-mass protoplanetary disks and Solar-system bodies, suggesting that our Solar-system may have originated in a low-density IRDC (Zeng, Jimenez-Serra et al., paper under review, submitted to A&A).

5) The detection of complex organic molecules in massive protostars may be produced by the interaction of high-density, low-velocity magneto-hydrodynamic shocks (Wang et al. 2016; Palau et al. 2017).

6) A quarter of the Cold Cores detected with the Plank satellite (belonging to the early release of the Plank Galactic Cold Cores catalogue) are excellent candidates to look for the initial conditions of massive star and star cluster formation (Zahorecz, Jimenez-Serra et al. 2016).

7) As part of the preparatory science for ALMA Band 2 and the SKA, I have detected a large number of complex organic molecules (carbon-chain species with more than 6 atoms in their molecular structure) in the Solar-system precursor L1544 (or pre-stellar core). These observations reveal a region located 4000 AU away with respect to the core's centre, which represents the complex organics reservoir in this pre-stellar core (Jimenez-Serra et al. 2016). I have contributed to the ALMA Band 2 White Book (Fuller et al. 2016).

8) I have also contributed to several studies of the chemistry of MHD shocks in molecular outflows in collaboration with the UCL Astrochemistry group, which have provided insight into the processes leading to the chemical complexity found in proto-Sun analogues. These studies have focused on the chemistry of key species for the origin of life, such as water, ammonia, sulphur and phosphorus (Holdship et al. 2016; Gomez-Ruiz et al. 2016; Lefloch et al. 2016).
Exploitation Route In the field of star formation, my observational work toward IRDCs is expected to provide additional constraints to theories of molecular cloud formation, allowing us to distinguish between flow-driven, cloud-cloud collisions and gravitational collapse scenarios for cloud formation. In particular, the detection of extended emission of SiO and CH3OH suggests that these massive clouds may have formed via cloud-cloud collisions.

My research regarding the 15N/14N isotopic ratio in IRDCs provides further evidence for the formation of the Solar System in a high-mass star-forming environment with several supernovae explosions. It also constraints the physical properties of such an environment with an average gas density lower than those typically found in IRDCs. These results will be relevant to researchers working in the origin and chemical composition of the solar nebula.

My work on the chemical complexity in pre-stellar cores will be used by the SKA Office to design and develop the capabilities of the SKA so that this kind of studies can be carried out once the SKA becomes operational. The ultimate goal of the project is to find amino acids at these early stages in the formation of a Solar-type system.
Sectors Education,Other

 
Description I was interviewed for the Jodcast by one of the PhD students at University of Manchester (webpage: http://www.jodcast.net/). I also participated in an Aspiration Day event at a local school to encourage children to carry out careers in science. I delivered a talk before a film screening for the Science Fiction Theater. This talk was recorded and it can be listened to at their podcast. Part of my research results will also be used by the engineers at the SKA Office to design and develop the capabilities of the observatory.
Sector Education,Other
Impact Types Societal

 
Description Jodcast (link available at http://www.jodcast.net/archive/201604Extra/) 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Last December I was interviewed for the Jodcast, an astronomy podcast from the University of Manchester's Jodrell Bank Observatory. The program is being prepared and it will be put on-line either this month or next month (based on the information I have from Jodcast). The potential impact of this activity is not known yet.
Year(s) Of Engagement Activity 2015
 
Description Participation in the Bluegate Fields Junior School Aspiration Day on 25/11/2016 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Aspiration Day was held on Friday 25th November 2016 and run from 1:30pm until 4:30pm in the Bluegate Fields Junior school hall. This was a kind of 'speed-dating' event where children came to different stations with different professionals (medical doctors, researchers, architects, etc...) and they could ask questions about our jobs. Each small group of 3 children stayed in each station for about 5 minutes before a bell sounded and they could move to another station. Parents also had the opportunity to come and speak to us in the hall by the end of the school day. Children were delighted with the event and they could learn how planets form and how life may have originated on Earth.
Year(s) Of Engagement Activity 2016
 
Description Talk at the Science Fiction Theater before a film screening (The Andromeda Strain) on 20th February 2017. The talk (titled "Life in Space?") was recorded and it is available at https://sciencefictiontheatre.co.uk/podcast/. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact I delivered a 10mintalk (titled "Life in Space?") before a film screening at the Science Fiction Theatre, located close to Dalston Junction in London. There were about 60 people attending the event and the talk was recorded. It can be listened to on their web podcast that can be found under this link: https://sciencefictiontheatre.co.uk/podcast/. People asked many questions and they came to talk/discuss with me after the talk.
Year(s) Of Engagement Activity 2017
URL https://sciencefictiontheatre.co.uk/previously/2017-2/the-andromeda-strain/