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Blue Stragglers in Dwarf Galaxies : A Hard Quest

Blue Stragglers in Dwarf Galaxies : A Hard Quest. GC are the “ default ” /reference environ. of BSS: hotter & bluer extension of normal MS TO mass of GCs is 0.8-0.9 Solar, and with ages of >10 Gyr, the detection of 1.2—1.5 solar nowadays is a unexpected

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Blue Stragglers in Dwarf Galaxies : A Hard Quest

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  1. Blue Stragglers in DwarfGalaxies: A Hard Quest

  2. GC are the “default” /reference environ. of BSS: hotter & bluer extension of normal MS • TO mass of GCs is 0.8-0.9 Solar, and with ages of >10 Gyr, the detection of 1.2—1.5 solar nowadays is a unexpected • Despite chemical anomalies and multiple populations, the BSS in GC are still a non-canonical population. • Origin of BSS : (A) mass-transfer in primordial binaries; or (B) collisional binaries due to dynamical collisions/encounters experienced by single/binary stars throughout the life of the cluster (DG).

  3. DG or Field stars ?...No…DG are separate class.

  4. Dwarf Gal. vs GC: • GC are natural envir of BSS, hence a reference point • Fundamental Plane relation over order of mag of mass show a division b/n 2 families: (i) Galaxies (ii) star clusters. • over several order of mag. the sizes of GC (3 pc) and DG (1000 pc) barely varies with mass. • Pre-SDSS: Classical DG have -9> Mv> -13. Masses up 10^7-8 Solar. • Post SDSS: Ultra-faint DG have Mv ~ -2 with Masses b/n 10^2-3…very interesting envir for BSS • Ultra-faint DG are the DM halos that are badly needed for CDM models. Indeed, they have M/L~1000 and are among the least evolved chemically • DG interesting for BSS must have nearby with no evidence of recent star forming activity. Misgeld & Hilker (2011)

  5. Avoid star forming galaxies

  6. Leo A dwarf Cole et al. (2007)

  7. Open clusters. Ideal cases:

  8. Open clusters. Sculptor dwarf Held et al. 2005

  9. Most interesting cases.

  10. Bootes dwarf Belokurov et al. (2006)

  11. Observational properties of BSS in DG: • Unlike in OC/GC: for DG, you have to first assume that the blue plume is made of pure BSS! Then cross your fingers • Radial distribution of BSS • Luminosity distribution of BSS • Specific frequency of BSS • BSS in the Galactic Bulge & Galactic Halo

  12. 1- BSS Radial distribution in DG: Mapelli et al. (2007) - However, survey a significant part of the DG is a must.

  13. 1- BSS Radial distribution in DG: Mapelli et al. (2007) • Account for DG ellipticity. • Account for DG foreground/background contamination.

  14. 1- BSS Radial distribution in DG: • In both DG: BSS are less concentrated than HB & RGB • In both DG: a hint of a max. rel. freq. b/n 1.5-2.5 r_c. • Observed distribution is hardly consistent with a central rise & more likely is a flat distribution. • Flat & not clumpy  most likely is a BSS ( less likely a young stell. pop.) • In both DG: wrt GC a total absence of a central peak in the rel. freq. of BSS in DG.

  15. 1- BSS Radial distribution in DG: Cetus & TucanaMonelli et al. (2012)

  16. 1- BSS Radial distribution in DG: Monelli et al. (2012) • For both DG: Absence of a strong central peaks • Cetus: wrt RGB,SGB the BSS distributionisconsistentlyflat, whilewrt HB ismarginallyflat. • Tucana: wrt SGB the BSS distributionismarginallyflat, whilewrt HB isnotconsistent with beingflat. • BSS/HB show largerfluctuations.

  17. Overall, the radial distribution of BSS in DG: • Is consistent with a flat distribution showing the absence of a central peak (as is in GC). • GC dynamical simulations (Mapelli et al. 2004, 2006) showed that the bimodal distribution can be reproduced only by requiring the central BSSs to be mainly COL-BSSs, whereas the peripheral BSSs to be MT-BSSs. • Granted the above  DG flat BSS distribution favors the MT-BSS. • Sollima et al. (2008) studying 13 low-density GC, conclude that the undisturbed evolution of primordial binaries could be the dominant BSS formation process in such low density environment.

  18. 2- BSS luminosity function in DG: • In GC: Bright BSS tend to be more concentrated than faint BSS. • Thiswasexplained with COLL-BSS beingbrighterthan MT-BSS, since COL-BSS conserve a largerfraction of the original mass of the collidingprogenitorswhereas MT-BSS is a relativelylessefficientprocess. • Thus: if the DG BSS are MT-BSS, then DG shouldnotshow anycorrelation b/n the BSS radialdistribution & theirbrightness.

  19. 2- BSS luminosity function in DG: Luminosity distribution of BSS in Draco & Ursa Minor. Blue histogram represents BSS with radial position r > rc while red histogram those r <rc [Mapelli et al. (2007)]….No differences.

  20. 2- BSS luminosity function in DG: Luminosity distribution of BSS in Cetus & Tucana. Long Dashed histogram represents BSS with radial position r > 1.5xrc while short-dashed histogram those r <1.5xrc [Monelli et al. (2012)]…No differences.

  21. Overall, the luminosity function of BSS in DG: • The luminosity function of BSS in GC shows radial dependence suggesting the coexistence of COL-BSSs and MT-BSSs. • In dSphs, COLL-BSS are hard to produce due to the low density, thus no radial dependence of the luminosity function is expected, and this is in full agreement with the observational evidence. • In Conclusion: the lum. function of BSS in DG argues in favor of MT origin

  22. 3- BSS frequency in GC: • GC: No correlation between BSS frequency (normalized to the HB) & the respective cluster collision rate. • Universal anti-correlation of the BSS frequency vs Mv • GC: Davies et al. (2004): while massive GC produce more coll. Binaries, they also destroy primordial binaries. • “…wide binaries destruction in high density environment GC causes the deficiency of BSS…” -Piotto et al. (2004) & De Marchi et al. (2006)

  23. 3- BSS frequency in DG: • The BSS frequency in dwarf galaxies, at any given MV , is always higher than that in GC of similar luminosities. • the BSS frequency for the ultra-faint-dwarf (UDF) is in excellent agreement with that observed in the Milky Way open clusters. • DG & OC might set an upper empirical limit for the BSS freq. • A statistically significantanti-correlation for dwarf galaxies, shallower than observed in GC • Anti-correlation is not expected should the blue plume contain young MS (if ever you’ll have a correlation). • Sgr ????

  24. Uncertain BSS freq. in SgrdSph: this is because a WFI 1 square degree field still covers less than ~6% of its stellar population, whereas for the remaining DG we manage to cover beyond the half light radius. Moreover, MW contamination is rather severe.

  25. 3- BSS frequency in DG: • Anti-correlation as a diagnostic: Carina dwarf galaxy is known to posses and intermediate and young age population, and as such is expectedly an outlier.

  26. 3- BSS frequency in DG: • Anti-correlation as a diagnostic: Carina dwarf galaxy is known to posses and intermediate and young age population, and as such is expectedly an outlier. • Bono et al. (2010)

  27. 3- BSS frequency in DG: • Anti-correlation as a diagnostic: Carina dwarf galaxy is known to posses and intermediate and young age population, and as such is expectedly an outlier. • Rizzi et al. (2003)

  28. 3- BSS frequency in DG: • Anti-correlation as a diagnostic: Canes Venatici I (Mv=-8.6, Martin et al. 2008) find a clumpiness of the young population, and conclude the presence of a ~2 Gyr population. • So…..Updating the plot for new dwarf galaxies:

  29. 3- BSS frequency in DG: • Uncertain cases: • U.Major II (Dall’Ora et al. 2012…..Mv=-4.2, F_BSS=~0.3) • [Fe/H]=-2.2, at D=32 kpc

  30. 3- BSS frequency in DG: • Uncertain cases: • Bootes II (Walsh et al. 2008, Mv=-2.7) • Distance 42 kpc, cannot be excluded as cluster on the verge of disruption. Yet, Koch et al. (2009) confirm a dSph nature.

  31. 3- BSS frequency in DG: • Uncertain cases: • Bootes II (Belokurov et al. 2009…Mv=-2.7) • Distance of 35 kpc

  32. 3- BSS frequency in DG: • Uncertain cases: • Willman I (Willman et al. 2011….Mv=-2.7)distance of 38 kpc.

  33. 3- BSS frequency in DG: • Fortunately there are 6 new DG entries: • Pis II (Mv=-4.1) (sand et al. 2012) at 183 kpc!!! • Leo V (Mv=-4.4) (sand et al. 2012) at 196 kpc!!! • CV II (Mv=-4.6) (sand et al. 2012) at 160 kpc • Leo IV (Mv=-5.0) Okamoto et al. (2012) at 154 kpc • Tucana (Mv=-9.5) (Monelli et al. 2012) at 887 kpc • Cetus (Mv=-10.1) (Monelli et al. 2012) at 755 kpc

  34. The statistical significance of the DG BSS freq. anti-correlation is solid. The prob. that a random sample of uncorrelated data-points would yield a correlation coef. F 0.0994 is less than 10^-6

  35. BSS freq. in UFD is now in full agreement with that observed in OC. Probably setting an empirical upper limit to BSS freq instellar systems. I am avoiding adding “the MW Halo” to the DG & OC….because: the MW Halo (on this diagram should have Mv b/n -17 & -18 and BSS freq. of around 0.2 (but as an upper limit).

  36. MW HALO: In this regards, the BSS freq. in low-density MW halo is of particular interest. But it is trouble! Preston & Sneden (2000) estimate N_BSS/N_BHB=5  horizontal green line, ~0.6….Mv? Red HB? 9k/5.6k  0.2 (See Santucci’s Poster)

  37. The star formation history of the MW cannot be very dissimilar from M31’s….& this diagram from Brown et al. (2008) shows clearly the presence of red HB in the very outskirts of M31 halo…Assuming both halos are similar, the BSS freq. in the MW should be lower than 0.2, because the addition of red HB would lower the overall freq.

  38. First detection of BSS in the Bulge (Clarkson et al. 2011) via proper-motion decontam. Bulge is old/young: chemical evidence suggests the stars mostly formed early and rapidly. While the majority view now is that most stars in the Boxy/Peanut bulge of the Milky Way (which we call here “the bulge”) formed ∼10 Gyr ago (e.g., Zoccali et al. 2003; Freeman 2008), the degree to which this star formation was extended over time, is presently only somewhat weakly constrained .Thus, better estimates of the young stellar population of the bulge, and thus its evolution, are a natural dividend of constraints on the BSS fraction.

  39. MW Bulge: can saturation lower the BSS freq.? • MW Halo: NHB = NBHB is adopted because only BHB can be distinguished kinematically and photometrically from the local disk population (Preston et al. 1991). • Back to MT/COLL BSS….

  40. Are there Coll-BSS in DG ? Most probably not. • DG with typical central lum. density of 0.006 Lsolar/pc^3 is several order lower than that in a GC reaching 8000 Lsolar/pc^3 lower dynamical evolution in DG and almost negligible collision factor. • Stellar specific coll.param. (estimated from the central surface density and the system core size) is a DG is 10^-5 times lower than in GC. This almost preclude the possibility of collisional binaries (due to dynamical collisions/encounters experienced by single/binary stars.

  41. Conclusions: • Just ~20 years ago: “This makes forming conclusion about the origin of BS in dwarf galaxies difficult, if not impossible, at the present time.” Stryker (1993) • Radial distribution & lum. function (as compared to the GC) clearly show the absence of a central peak or radial dependence of the BSS lum., both argue in favor of MT-BSS origin. • Specific freq. is quantitatively derived for a significant sample of non-star-forming galaxies, and it shows: • (i) the dwarf galaxies BSS frequency is always higher than in GC (predicted already in Renzini et al. 1977). • (ii) at the low-luminosity regime, DG & OC probably set a realistic upper limit to the BSS freq. • (iii) with respect to DG and GC: the BSS freq. of MW halo & bulge are ”interesting” • (iii) the low-density of DG & their low encounter factor offer a friendly environ. for primordial binaries to survive & evolve (AC). “..The depletion of binaries by encounters produces the anti-corr b/n number of AC (and or BSS)… ”

  42. Notes…..

  43. Evolved- BSS or Vertical Clump?

  44. BSS progeny in DG: 1.0, 1.2 and 1.5 Gyr, [Fe/H]=-1.3 isochrones (Girardi et al. (2002). Vertical clump (Gallart et al. 2005): few hundred Myr to ~ 1 Gyr old, helium-burning stars.

  45. Vertical clump stars or Evolved- BSS ? For GCFerraro et al. (1999): BSS/evolved-BSS ~ 7, consistent with that for LeoII dShph (~8)….Thus, there is agreement for the BSS/Evolved-BSS ratios in both GC and DG…

  46. BSS progeny in DG: Theoretical prediction based on the SFH of the 2 galaxies (Cetus & Tucana) show that BSS vs VC rations are ~8. This is again in excellent agreement with BSS/VC star counts (~10) for both galaxies, and points to the fact that the selected VC stars are evolved counter-parts of the BSS. AC: in both galaxies are bluer and brighter than the bulk of the evolved-BSS. AC might have larger masses than a typical BSS. Predicted AC/BSS ratios vary b/n 1-10, but for both DG it is much lower than 1….Uncertainties due to the metallicity ?!?!, work in progress.

  47. MT-BSS & AC: • In GC/DG systems, Ad is larger than the separation needed to produce an AC • GC with AC are those with lowest densities and closest to the DG. • Scarcity of AC in GC results from the shrinkage of the orbits to separation that is too small to produce AC (<=0.25 AU). If they shrink to this level (4 stellar radii) they collide • The depletion of binaries by encounters produces the anti-corr b/n number of AC (and or BSS) and the degree of central concentration of the stellar system. • Renzini et al. (1977)

  48. SxPhoenicis in DG: • Pop. II variables that exhibit short period (P<0.1 day as in GC) with spectral classification A2-F5, and show variations b/n 0.001 up to several 0.1 mag. • Their variability has a Period/Luminosity relation, and all known Sx Pho in GC are BSS and they show large diversity in their pulsation forms Poretti et al. (2008)

  49. Sx Phoenicis in DG: • They detect a large scatter in the P-L relation. The identification of different pulsation modes (the brighter stars as first-overtone pulsators) reduces the scatter, however, it is the group of sub-luminous (below the P-L relation) at shortest periods that remained puzzling and they propose that these are the merging product of close-binaries (COLL-BSS)….Although this was a speculative scenario, similar sub-luminous stars were found in the Carina dwarf, as well. More data needed…

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