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A NEWLY DISCOVERED STELLAR TYPE: DUSTY POST-RED GIANT BRANCH (POST-RGB) STARS IN THE MAGELLANIC CLOUDS Devika kamath Institute of Astronomy, KU. Leuven, Belgium Collaborators (2) Peter Wood , Hans Van Winckel ,Jundan Nie (1) (1) ! RSAA, ANU, Australia IvS, KU.Leuven, Belgium (3) Beijing Normal University, China (2) th 10 June, 2015 PHYSICS OF EVOLVED STARS, 2015, Nice In Memory of Olivier Chesneau… Weather = ☀ ️, blue skies, NICE! EVOLUTIONARY FATE OF A SINGLE STAR Nuclear burning lives are terminated on the AGB with a ‘Superwind’ mass-loss (mass-loss rates up to 10 M yr ) −4 ⊙ −1 Surface Enrichment, Variability & Massloss C(Tc)" Core He Flash 108 years S(Tc)" M" 2nd$Dredge) Up$ 1010 years FGB$ K" Ist$Dredge-Up$ G" Heavy$element$ Nucleosynthesis$ /3rd$Dredge-Up$ C/O,$[s/Fe]$ AN ALTERNATIVE EVOLUTIONARY CHANNEL Great majority of PNe are not spherical: axi-symmetry; point-symmetry jet-like structures are common… NOAM :) (Balick & Frank, 2002; De Marco 2008, Zijlstra 07) BINARITY! Binary interaction can determine the ultimate of the star … GL ES TA RS D N I W C C A RIE S R T S M ROCHE LOBE OVERFLOW N O I T RE MASS-LOSS LON GP AS ERI OD R E SF AN MERGERS R E L B A ST UN Y CLOSE PERIOD BINARIES CO M MO NE NV ELO P E E A L U B E N Y R A T E N A L P JETS UB LE -D EG EN RA TE SIN LA ERICI T DO T A I ED M BI ER T PO IN A-SPH BIN A D O I R E P E S E I R A BIN WHEN DOES THE STAR FILL IT’S ROCHE LOBE??? DISCLAIMER: THE PHYSICS OF THE POST-ROCHE LOBE FILLING EVOLUTION ARE VERY UNCERTAIN… WHEN DOES THE STAR FILL IT’S ROCHE LOBE??? Stars reach the tip of the AGB (TAGB) without filling its Roche lobe… Tip-AGB OUTCOME ~ they likely evolve as single stars do RESULT ~ formation of a PN with wide orbits (Moe & De Marco 2006) WHEN DOES THE STAR FILL IT’S ROCHE LOBE??? Roche lobe filling occurs on the AGB but above the RGB-tip… OUTCOME 1 ~ common envelope (CE) event RESULT ~ close binary or stellar merger (Ivanova et al. 2013) OUTCOME2 ~ some sort of a stable mass transfer RESULT 2~ Formation of an intermediate period binary e.g.: Post-AGB binaries surrounded with DUSTY circumbinary discs (Van Winckel 2007) WHEN DOES THE STAR FILL IT’S ROCHE LOBE??? Roche lobe filling occurs below the RGB-tip… (e.g., Han et al. 1995; Heber 2009; Nie et al. 2012) OUTCOME 1 ~ common envelope (CE) event RESULT ~ close binary or stellar merger (Paczynski 1976; Webbink 1984) OUTCOME2 ~ some sort of stable mass transfer RESULT 2 ~ Formation of an intermediate period binary e.g., ”Post-RGB” binaries surrounded with DUSTY circumbinary discs!!! THE DISCOVERY OF DUSTY POST-RGB STARS… OPTICALLY VISIBLE POST-AGB STARS IN THE SMC* & LMC** *Kamath et al. 2014 MNRAS, 439, 2211 **Kamath et al. 2015 MNRAS (Accepted) Mid-IR Spitzer Space Telescope Surveys ! Candidates with Mid-IR excess selected from the Mid-IR SST survey SMC: S3MC (Bolatto et al. 2007) & SAGE-SMC (Gordon et al. 2010) LMC: SAGE (Meixner et al. 2006) & (Blum et al. 2006) ✓ Candidate Selection ✓ Spectroscopic Examination ✓ SED Analysis ✓ Variability Analysis ✓ Spectroscopically verified Catalogues of Post-AGB, “Post-RGBs* and other interesting objects 1) CANDIDATE SELECTION: Mid-IR excess Optical colour Suitable luminosity 2) OPTICAL SPECTROSCOPY AAOmega on the 3.9m AAT Optical Low Resolution Spectra R~1300 Wavelength Coverage = 3700 Å - 8700 Å SMC LMC A NEWLY-DISCOVERED STELLAR TYPE: DUSTY POST-RGB STARS LMC SMC ✓Mid-IR excess ✓A- K spectral 42 Post-RGBs types ✓Low log g ✓low [Fe/H] ✓low -luminosity (< 2500 L⊙) 119 Post-RGBs Note: These numbers are not complete due to incompleteness of the survey… MINIMUM Expected numbers: SMC ~ 30 more, LMC ~ 750 more WHAT ARE THESE POST-RGB ??? S • Pre-mature evolution off the RGB via massloss • Single star mass loss • • Mass loss induced via binary • Post RGBs too weak! Way to go! • Very like to be Binaries! • Can they be Mergers… ? INTERLOPING OBJECTS IN OTHER EVOLUTIONARY STAGES… • Core He Burning ? • Post RGBs No! Too Dusty… INTERLOPING OBJECTS IN OTHER EVOLUTIONARY STAGES… Pre-main sequence? N • Post RGBs • No! • Low Log g and [Fe/H] 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 LMC 3.0 2.5 2.0 1.5 1.0 0.5 0.0 [Fe/H] 0.5 1.0 The Post-RGB stars (old) have [Fe/H] peaking at about -1.0 dex The Post-AGB stars (old) have [Fe/H] peaking at about -0.7 dex The PMS are a younger population peaking at >-0.5 dex INTERLOPING OBJECTS IN OTHER EVOLUTIONARY STAGES… • EAGB RGB Early AGB stars? • Initial masses M <1.85 Msun • • Unlikely! Initial masses M <1.85 Msun • Maybe!!! Binary interaction depends on when the star attains its ‘largest radii… Establish connections to possible precursors and progeny…. ESTABLISHING THEIR EVOLUTIONARY STATUS - PRECURSORS SEQUENCE-E Variables Close binary red giants that show ellipsoidal light variations Nicholls et al. 2010 Luminosities of the TAGB Close binary Close binary PNNe have AGB luminosities above the TRGB, EAGB and RGB binaries undergoing a CE event but not merging have luminosities below the TRGB Soszynski & Wood 2012 Nie et al., 2012 Mbol(TRGB) = −3.6 EVOLUTIONARY CONNECTION BETWEEN THE SEQUENCE- E STARS AND POST-RGB STARS Method: Comparing theoretically predicted birthrates Nie et al. (2012) with the observationally determined birthrates of our new sample of dusty postRGB stars Log L/Lsun Stars evolving off the RGB Tip-RGB Stars evolving along the RGB Teff 2.77 x 106 y ear s Tip-RGB dt = TOP 1 MAGNITUDE OF THE RGB Log L/Lsun THEORETICAL BIRTHRATES… RGB Teff Relative theoretical birthrates (PRGB, PEAGB, MERGERS, tip-RGB) in arbitrary units, come from population synthesis models, Nie et al, 2012 BUT WE NEED TO SCALE THEM TO THE OBSERVATIONAL FIELDS OF THE POST-RGB STARS The number of stars we expect to see at any given time in the top 1 magnitude of the RGB is k = 2.77 x106 × birthrate tip-RGB Total number of stars observed in the top 1 magnitude of the RGB in the fields searched for post-RGB stars is 118927 (from SAGE) Total predicted birthrates = 118927*(birthrate/k) OBSERVATIONAL BIRTHRATES OF POST-RGB STARS Number of observed post-RGB stars Estimated evolutionary lifetime RGB log Teff range = log Teff(RGB) + 0.05 and continues to log Teff = 4. Note: We take into account the incompleteness of the survey… (incompleteness factor of 7.3) CONNECTION TO POPULATION MODELS OF RGB BINARIES Post-RGB production rates Post-EAGB production rates Birthrates of mergers on the RGB Total predicted rate of production OBSERVED BIRTHRATES Ratio of the observed to the total predicted birthrate CONNECTION TO POPULATION MODELS OF RGB BINARIES Observationally estimated postRGB birthrate is much higher than the theoretically predicted birthrate ! The average ratio of observed to predicted birthrate is 9.6 At lower metallicities, mergers dominate Post-EAGB birthrates increases to about 25% of the total birthrate at the highest luminosities A RE th es e th e M ER G ER S ? OBSERVATIONAL BIRTHRATES OF POST-RGB STARS Number of observed post-RGB stars Estimated evolutionary lifetime RGB log Teff range = log Teff(RGB) + 0.05 and continues to log Teff = 4. Note: We take into account the incompleteness of the surgery…. UNCERTAINTIES • An over-estimation of the incompleteness factor ! • An underestimate of the post-RGB evolution time ! • Interlopers in the post-RGB sample ! • Uncertainties in the model post-RGB birthrate EVOLUTIONARY STATUS -PROGENY Sub-dwarf B stars Binary He WDs/ Cataclysmic Variables Vos et al., 2013, TALK OF JORIS VOS… coming soon!!! Low-luminosity Planetary Nebulae Merle. T et al., 2014 (Bond & Livio 1990; Yungelson et al. 1993; Soker 1997; Bond 2000; Zijlstra 2007; de Marco 2009) (Bond 1994, 2000; Miszalski et al. 2009) CONCLUSIONS • • Newly discovered low-luminosity, dusty post-RGB stars • An unexplored phase of binary stellar evolution • Termination of RGB evolution via binary interaction Precursors - Sequence-E stars • The observationally estimated post-RGB birthrate is much higher than the theoretically predicted birthrate • Some of these objects are likely to be products of mergers. Models predict that mergers dominate at lower luminosities ! • Progeny - Binary He WDs, SdBs, Low-luminsoity PNe??? CONCLUSIONS • • Newly discovered low-luminosity, dusty post-RGB stars • An unexplored phase of binary stellar evolution • Termination of RGB evolution via binary interaction Precursors - Sequence-E stars • R I B ‘ Y P P A H The observationally estimated post-RGB birthrate is much higher than the theoretically predicted birthrate • ! • R Y A D ’ D Some of these objects are likely to be products of mergers. Models predict that mergers dominate at lower luminosities Progeny - Binary He WDs, SdBs, Low-luminsoity PNe??? ! ! ! ! ! V E E J A ESTABLISHING THE BINARY NATURE OF THESE POSTRGB SYSTEMS Radial Velocity Estimates • On-going… SED Characteristics • • l Fl [Watt/m2 ] • J051942.32-690535.4 10 14 10 15 10 16 10 Majority of them show disc-like SEDs (=> binaries) but a small number show shell-like SEDs (=> evolved discs with cooler dust???) E(B-V) = 0.12+0.04 0.04 Teff = 4500 ± 250 Disc-type SED with hot dust 1 100 101 102 l [µm] J045539.61-681202.9 E(B-V) = 1.19+0.01 0.02 l Fl [Watt/m2 ] • Post-RGB SEDs 10 14 10 15 10 16 10 Teff = 7625 ± 250 1 100 101 Disc-type SEDs (evolved discs) 102 l [µm] Chemical Abundance Analysis J045815.58-662507.0 E(B-V) = 0.06+0.03 0.04 • On-going… BUT challenging as these objects are rather faint… l Fl [Watt/m2 ] • 10 15 10 16 10 Teff = 5303 ± 250 Shell-type SED? 1 100 101 l [µm] 102 GALACTIC POST-RGBS??? The poorly constrained distances to Galactic objects currently classified as postAGB stars means that it is not possible to identify post-RGB systems among them… POSSIBLE GALACTIC POST-RGBS I : ST PUP • Chemical pattern: Depletion (characteristic of binaries) • Pop II Cepheid - W-Vir star with (P ~19 days) Gonzalez & Wallerstein et al 1996 POSSIBLE GALACTIC POST-RGB II : AU PEG • Pop II Cepheid - W-Vir star with pulsation period of 2 days… • Mass function of 0.57 Msun (m1 ~0.3 to 0.65 Msun) and (m2 ~ 0.9 to 1.4 Msun) • Chemical pattern: No signs of depletion Jurkovic M. et al., 2007