Molecular richness in protostars: a paradise for astrochemists

Copyright © 2009-2017 IRAM Ana López Sepulcre Institut de Planétologie et d’Astrophysique de Grenoble (FR) Institut de Radio Astronomie Millimétrique (FR) Garching, 11 March 2025 Molecular richness in protostars A paradise for astrochemists solar-mass T < 10 K n > 105 cm-3 Molecular envelope ~0.1 pc ~20000 au CO freeze-out D-enhancement Collapse Envelope dissipation Grain coagulation Planet formation ~2000 au Sketches kindly provided by Marta de Simone ~50 au ~1 au Pre-stellar core Protostellar core Protoplanetary disk Main sequence star Review by Podio Review by Podio 1. What is the typical molecular composition in protostars? 2. Protostellar chemical diversity: why, and (how) does it depend on the environment? 3. Takeaway messages, open questions, and steps forward Protostellar core: Onset of star formation Hot corino (?): H2O ice sublimation “complex” organics Outline Disclaimer: As a human being, I am limited and biased What follows is not exhaustive! 2. Protostellar chemical diversity: why, and (how) does it depend on the environment? 3. Takeaway messages, open questions, and steps forward Protostellar core: Onset of star formation Hot corino (?): H2O ice sublimation “complex” organics Outline 1. What is the typical molecular composition in protostars? ** Spoiler: There is no such thing as “typical”, diversity rules! ** Main focus on organic chemical budget IRAM 30-m Protostellar organic richness: IRAS 16293-2422 Cazaux et al. 2003 TIMASSS spectral survey: Caux et al. 2011 SINGLE DISHINTERFEROMETRY PILS spectral survey: Jørgensen et al. 2016 ALMA IRAM 30-m Extremely line-rich protostellar spectra Most of the lines are weak and emiKed by organic molecules • Compact (< 100 au) • Hot (T > 100 K) and dense (n > 107 cm-3) • Rich in interstellar Complex Organic Molecules (iCOMs) Hot corinos HCOOH: formic acid CH3CHO: acetaldehyde CH3OCHO: methyl formate CH3OCH3: dimethyl ether CH3COCH3: acetone CH2OHCHO: glycolaldehyde C2H5OH: ethanol CH3CN: methyl cyanide C2H5CN: ethyl cyanide NH2CHO: formamide CH3OH: methanol iCOM dicYonary CO CO H2O H2O HCOOCH3 Dust grain CH3OH H2CO H2O CO CO CO H2O H2O H2O Dust grain HCO H2CO H2O CO Bianchi et al. 2019 Ceccarelli et al. 2017 IRAS4A ConYnuum @ 1.3mm NOEMA CLASS 0 CLASS 1 Resolving iCOM emission in hot corinos Lee et al. 2022 (see also Okoda et al. 2022) CH3OH HH212 NH2CHO ALMA • StraYficaYon of iCOM emission in hot corinos reflect their different binding energies, i.e. the different temperatures at which they desorb from grain surfaces Ferrero et al. 2020 (see also Wakelam et al. 2017, Das et al. 2018) Resolving iCOM emission in hot corinos Lee et al. 2022 (see also Okoda et al. 2022) CH3OH HH212 NH2CHO • StraYficaYon of iCOM emission in hot corinos reflect their different binding energies, i.e. the different temperatures at which they desorb from grain surfaces ALMA Frediani et al. 2025 IRAS 4A2 CH2OHCHO C2H5OH CH3CHO HCOOCH3 CH3OH • MulY-line analysis and spaYal resoluYon of several straYfied hot corino iCOMs can help constrain the temperature profile ALMA • This straYficaYon strongly suggests that iCOMs can be found in solid form at lower T Identification of solid-state iCOMs in protostars JWST is showing the first promising results of iCOMs on ices and much more will be elucidated in the coming years Is this proof that iCOMs form on the surface of dust grains? Rocha et al. 2024 IRAS 2A JWST Sakai et al. 2008 IRAM PdBI NRO 45-m & GBT • Warm protostellar envelope rich in carbon chains 10 to 100 Ymes more abundant than in hot corino sources • No evidence of saturated iCOMs • Icy mantles contain large amounts of CH4 CH4 Dust grain CO T > 25K CH4 CH4 CH4 CO CH4 A chemically different type of protostar: Warm Carbon Chain Chemistry (WCCC) object Sakai et al. 2010 The “unsuccessful” search for iCOMs in L1527 Hot corinos versus WCCC protostars • Hot corinos: Complex Organic Molecules (iCOMs) e.g. HCOOCH3, CH3OCH3, C2H5CN Abundances ~100x those in WCCCs Sizes < 100 au Example: IRAS 4A • WCCC protostars: Unsaturated (long) carbon chains e.g. C4H2, HC5N, C4H, C6H Abundances ~10-100x those in hot corinos Sizes ~2000 au Example: L1527 see e.g. Yoshida et al. 2019 for an abundance comparison between hot corinos and WCCC objects WCCC structure ~2000-3000 au Inner region T>100 K H2O sublimation iCOMs deficiency WCCC zone (CH4-rich) T > 25 K CH4 sublimation enriched in Hydrocarbons (e.g. CCH, C4H, c-C3H2) Surrounding molecular cloud Hot corino structure ~150 au Hot corino region T>100 K H2O sublimation enriched in iCOMs (e.g. CH3OH, CH3CHO, CH3OCH3) Envelope (CH4-poor) Hydrocarbons deficiency Surrounding molecular cloud Courtesy of M. Bouvier Courtesy of M. Bouvier © NASA, ESA, CSA, and STScI. (JWST) 1. What is the typical molecular composition in protostars? 2. Protostellar chemical diversity: why, and (how) does it depend on the environment? 3. Takeaway messages, open questions, and steps forward Protostellar core: Onset of star formation Hot corino (?): H2O ice sublimation “complex” organics Outline WCCC (?): CH4 ice sublimation C-chains Chemical diversity explained (?) WCCC (C-chains) C —> ice: CH4 Pre-stellar duraRon Rmescale Short (~105 yr) Hot corino (iCOMs) CO —> ice: CH3OH Long (~106 yr) (Sakai & Yamamoto 2013) Pre-stellar environmental effects Environmental factors that may affect the grain mantle composiYon inherited by the protostellar phase are density, temperature, and UV/cosmic-ray irradiaYon (e.g. Spezzano et al. 2016, 2020, Aikawa et al. 2020, Kalvans 2021) CH4 grain CO T > 25K CH4 CH4 CH4 CO CH4 CO H2O H2O CH3OH grain CH3OH H2CO H2O CO T > 100K C C H2O H2O grain CO H2O CO CH4 CH4 CH4 CO CO H2O H2O grain H2CO H2O CH3OH CH3OH CH3OH Astrochemical modelling finds that WCCC is favoured by a long pre-stellar cold phase (Aikawa et al. 2020) Hybrid protostars, presenYng both hot corino and WCCC behaviours, were intuiYvely predicted ~10 yrs ago Oya et al. 2017 HOWEVER: • Only 4 hybrid protostars “solidly” idenYfied to date (Oya et al. 2017, Imai et al. 2022, Okoda et al. 2023) • C-chains/rings not yet well characterised chemically The middle point: hybrid protostars Graninger et al. 2016 B335 ALMA Imai et al. 2016 First hybrid protostar confirmed in 2016, in a Bok Globule Bok Globules/isolated protostars tend to show hybrid behaviour What about other environments? Orion versus Perseus: environment matters ORION PERSEUS VS Analogue of the Solar System’s natal environment (see Adams et al. 2010, Pfalzner et al. 2015, Pfalzner & Vincke 2020) Perseus Molecular Cloud Loose proto-cluster! Less UV-illuminated PEACHES 50 solar-mass protostars ORANGES 19 solar-mass protostars OMC-2/3 Dense protocluster! Highly UV-illuminated 31% hot corinos are scarce 56% hot corinos are abundant Yang et al. 2021 See also Higuchi et al. 2018 Credit: ESO Credit: Gerhard Bachmayer Bouvier et al. 2022 But see also Hsu et al. 2022 What about WCCC statistics? • Neither PEACHES nor ORANGES were designed to adequately probe WCCC tracers and spaYal scales. 2017 2025 # hot corinos 9 ~50 # WCCC objects 5 6 This is a cell membrane, whose composiYon includes long hydrocarbon chains A complete picture of organics in protostars should include WCCC studies • ObservaYons have so far been heavily biased towards hot corinos due to their enrichment in iCOMs and their interest as building blocks of prebioYc molecules Notes of caution on WCCC surveys The detecYon of small C-chains/rings such as CCH and c-C3H2 in protostars cannot be automaYcally aKributed to WCCC, which is triggered by warm (>25 K), sublimated CH4. These species can also be efficiently synthesised in photo-dissociaYon regions (PDRs). 1. Single-dish surveys using CCH or c-C3H2 to idenYfy WCCC protostars should be avoided. Instead, interferometric maps should be privileged to assess their spaYal distribuYon concentrated in inner warm envelope around the protostellar object. (see Bouvier et al. 2022) 2. A secure WCCC idenYficaYon requires the detecYon of several, heavier C-chains such as C4H, C4H2, HC5N…, and the characterisaYon of the emiung gas via mulY-line radiaYve transfer analysis. Imai et al. 2016 B335 WCCC does not exist, we are just seeing UVilluminated outflow cavity walls! (some people in the community) ALMA Towards improving WCCC statistics • Follow up of the hot corino hunt carried out with the ORANGES survey using both ALMA (3mm and 7mm) and NOEMA (3mm) Main targeted species: C3H, C4H, C4H2, C3H2, CH3CCH, HC3N, HC5N WCCC structure ~2000-3000 au Inner region T>100 K H2O sublimation iCOMs deficiency WCCC zone (CH4-rich) T > 25 K CH4 sublimation enriched in Hydrocarbons (e.g. CCH, C4H, c-C3H2) Surrounding molecular cloud Analogue of the Solar System’s natal environment • Other regions and parYcularly isolated protostars are very promising candidates for WCCC/hot corino studies: are they all hybrids? • Comparison with CH4/CH3OH solid abundance raYos? JWST/ALMA+NOEMA+JVLA… 1. What is the typical molecular composition in protostars? 2. Protostellar chemical diversity: why, and (how) does it depend on the environment? 3. Takeaway messages, open questions, and steps forward Protostellar core: Onset of star formation Hot corino (?): H2O ice sublimation “complex” organics Outline WCCC (?): CH4 ice sublimation C-chains