Star formation in the Local Group Guido De Marchi (ESA), Nino - PowerPoint PPT Presentation


 Star formation 
 in the Local Group Guido De Marchi (ESA), Nino Panagia (STScI) 
 Elena Sabbi (STScI), Giacomo Beccari (ESO) and NIRSpec GTO team ! 1

Motivation • Solar mass stars account for most of the star formation in galaxies • Low mass stars can form in small clouds as well as in big ones, and form on longer timescale than massive stars • Need to probe diverse environments, Magellanic Clouds crucial for metallicity • At redshift z~2, environment was similar to Magellanic Clouds ! 2

PMS stars: How to find them? Low-mass stars grow in mass over time through accretion of matter from a circumstellar disc (e.g. Lynden-Bell & Pringle 1974; Bertout 1989) Typical signature : UV, IR and H α excess emission How to measure it? Need spectroscopy Well calibrated relationship between L(H α ) and L acc

Spectroscopic search signs of 
 accretion ! 4

Until JWST only feasible in the Milky Way 


In the meanwhile … Powerful method combines HST broad- ( V, I ) and narrow- band ( H α ) photometry and allows us to: • identify all objects with H α excess emission 
 • derive accretion luminosity and mass accretion rates 
 • for hundreds of stars simultaneously De Marchi, Panagia & Romaniello 2010, ApJ, 715, 1 
 Beccari, Spezzi, De Marchi et al. 2010, ApJ, 720, 1108 
 De Marchi, Panagia & Sabbi 2011, ApJ, 740, 10 
 De Marchi, Panagia, Romaniello et al. 2011, ApJ, 740, 11 
 Spezzi, De Marchi, Panagia et al. 2012, MNRAS, 421, 78 
 De Marchi, Beccari & Panagia 2013, ApJ, 775, 68 
 Beccari, De Marchi, Panagia et al. 2015, A&A, 574, A44 De Marchi, Panagia & Beccari 2017, ApJ, 846, 110 Biazzo, Beccari, De Marchi, Panagia 2019, ApJ, 875, 51 ! 6

Actively accreting PMS stars These PMS stars are discovered from broad-band (V, I) and narrow-band (Hα) • photometry and show strong Hα excess emission due to ongoing accretion De Marchi et al. 2011a, 2017 De Marchi et al. 2011b, 2011c, 2013 gives L(H α ) 30 Dor (LMC) NGC 346 (SMC) In massive young clusters in the local group thousands PMS stars continue 
 • to accrete much longer than the few Myr typical of nearby associations ! 7

Hodge 301 HTTP (Sabbi et al. 2014, 2016) Accretion rate and metallicity 2 3 1 5000 10000 15000 20000 NGC 2060 4 5 6 Y 7 8 9 R 136 ~ 200 pc 5000 10000 15000 20000 25000 30000 X

H α photometry De Marchi, Panagia & Beccari 2017 R136 Cluster within r < 20 pc ~1,000 stars ! 9

H α photometry De Marchi, Panagia, Sabbi, et al. (in prep) Tarantula 
 Nebula within r < 120 pc ~14,000 stars ! 10

PMS stars in the Tarantula De Marchi, Panagia, Sabbi, et al. (in prep) Tarantula ! 11

11 14,000 and counting … De Marchi, Panagia, Sabbi, et al. (in prep) Tarantula 2 Myr 4 8 A V =1 16 32 ! 12

Reddening vector in all bands De Marchi et al. 2016 UV U V I J H ! 13

Extinction law at optical and infrared Not just a few lines of sight, but thousands! • Additional grey component, excess of big grains • De Marchi & Panagia 2019, 2014; De Marchi et al. 2016 Galactic ISM ! 14

11 14,000 and counting … De Marchi, Panagia, Sabbi, et al. (in prep) Tarantula 2 Myr 4 8 A V =1 16 32 ! 15

12 14,000 and counting … De Marchi, Panagia, Sabbi, et al. (in prep) Tarantula < 8 Myr > 16 Myr ! 16

N E H301 R136 NGC 2060

< 8 Myr N E

N > 16 Myr E

Multiple generations ubiquitous De Marchi et al. 2013 De Marchi et al. 2011b NGC602 in SMC NGC346 in SMC Multi-generation pattern always seen, Δ t ~ 10 Myr • • Younger PMS stars always more concentrated • Older PMS stars always more widely distributed ! 18

Accretion evolution with time De Marchi, Panagia, Beccari 2017 R 136 ! 19

Accretion evolution with time Sicilia-Aguilar et al. 2006; 2010 Hartmann et al. 1998 ! 20

Accretion evolution with time De Marchi, Panagia, Beccari 2017 R 136 ! 21

Accretion evolution with time De Marchi, Panagia, Beccari 2017 R 136 ! 22

Accretion evolution with time De Marchi, Panagia, Beccari 2017 R 136 ! 22

Accretion rate and metallicity De Marchi, Panagia & Beccari 2017 SMC LMC LH 95 MW ! 23

N E

! 25 H α variability NE field NW field 10 σ V SE field SW field

H α variability De Marchi, Panagia, Sabbi (in prep) 
 Stars with 5 
 H α epochs H α fluctuation [mag] • < 8 Myr • 8–16 Myr • > 16 Myr 
 bright faint H α brightness [mag] ! 26

H α variability < 8 Myr > 16 Myr ! 27

Richert, Lyra & Kuchner 2017

H α variability: a Poisson toy-model De Marchi, Panagia, Sabbi (in prep) 
 
 H α fluctuation [mag] • < 8 Myr • 8–16 Myr • > 16 Myr 
 H α brightness [mag] ! 29

H α variability: a Poisson toy-model De Marchi, Panagia, Sabbi (in prep) 
 
 Average accretion in Mirandas per week H α fluctuation [mag] • < 8 Myr • 8–16 Myr • > 16 Myr 
 1 4 20 64 Miranda: ~ 5 x 10 –11 M ◉ H α brightness [mag] ! 29

6 Looking ahead with JWST NGC 3603 (MW) 30 Dor (LMC) NGC 346 (SMC) N N E E Spectra of ~100 stars per field, easy with NIRSpec: R ~ 1000–2700, 1.7 – 3.0 μm, include Pa α , Br β , Br γ Photometry of thousands of stars in Pa α , Br α with NIRCam ! 30 Rich sample of younger and older PMS stars

7 Fitting targets in microshutters

7

13 Observations: spectral features Pa α K AB ~ 22 Br γ Br β G235M (R ~ 1000) for line luminosity G235H (R ~ 2700) for gas kinematics ! 32

Coordinated parallels NIRCam will cover fields about 7.5 arcmin from NIRSpec, still plenty 
 of star formation going on in those regions! Broad- and narrow-band (Pa α , Br α ) imaging to identify PMS stars that are accreting. Same as we did with HST H α photometry. Pa α Br α F150W F277W F182M F430M ! 33

Summary • Multi-generation patterns common in all regions, Δ t ~ 10 Myr, younger generations are always more concentrated • Extinction in starburst cluster is temporarily altered by SNe-II for Δ t ~ 50 – 100 Myr after star formation episode • Mass accretion rate depends on metallicity, at low metallicity stars accrete more and longer, sizeable fraction of stellar mass accreted during PMS phase • Accretion process is discrete and made up of a number of clumps all with a similar mass (~ Miranda) 
 ! 34

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