The Case of the Missing Binaries

Transcription

1 The Case of the Missing Binaries L. Aguilar (UNAM/México) B. Carney (U. of North Carolina) D. Latham (Center for Astrophysics) J. Laird (Bowling Green State U.)

2 Antecedents of the case

3 Antecedents of the case Binary and multiple stars are common in the solar neighborhood Distribution of semimajor axes Heacox & Gathright (1994) Data from Duquennoy & Mayor (1991)

4 Antecedents of the case Binary and multiple stars are common in the solar neighborhood The first studies of high-velocity, low-metallicity stars, suggested that the halo was deficient in binaries e.g., Abt & Levy (1969), Crampton & Hartwick (1972), Abt & Willmarth (1987)

5 Antecedents of the case Binary and multiple stars are common in the solar neighborhood The first studies of high-velocity, low-metallicity stars, suggested that the halo was deficient in binaries The situation changed in the 80's when more binaries were discovered in the halo e.g., Mayor & Turon (1982), Greenstein & Saha (1986), Jasniewicz & Mayor (1986), Lindgreen et al. (1987)

6 The evidence

7 The evidence In the mid 80's, Carney, Latham, Laird and Aguilar (CLLA) began a long-term program of spectroscopic monitoring and study of high-velocity stars in the Solar neighborhood The CLLA team Carney & Latham (1987), Laird et al. (1988), Carney et al. (2001)

8 The evidence The sample contains 1,464 stars selected from the Lowell and NLTT proper motion surveys.

9 The evidence All stars have been monitored for almost 20 years. Our goal has been to get at least 10 velocity measurements over a span of at least 10 years. Temporal coverage Radial velocity errors as a function of metallicity

10 The evidence Metallicity, position and kinematics have been obtained for each star in the sample. The sample safeguarded in paperware

11 The evidence The three components of galactic velocity are determined for each star. LSR U O = -10.2, V O = +15.1, W O = +7.4 km/s V circ = -220 km/s V rot = 0 (inertial) U W V

12 The evidence Metallicities are obtained by χ 2 matching with an atlas of stellar synthetic spectra Carney et al. (1994) Metallicity sequence [Fe/H]

13 The evidence The final sample used in this investigation contains 756 isolated stars and 238 binaries. Subgiants, blue stragglers and other stars of uncertain metallicity, or kinematics, were left out for this investigation Metallicity vs V velocity for the sample

14 The evidence The final sample used in this investigation contains 756 isolated stars and 238 binaries. Subgiants, blue stragglers and other stars of uncertain metallicity, or kinematics, were left out for this investigation For 200 binaries the orbit has been determined. Orbital periods range from 1.9 to 7,500 days (20.5 years). Assuming a total mass of 1.2 M o for the binaries, this implies separations of 0.03 to 8 A.U. Metallicity vs V velocity for the sample

15 From the beginning, we noticed a lack of binaries for stars on extreme retrograde motion (V<300 km/s). The evidence + Single stars o Binaries

16 The investigation begins Having detected the binary deficiency, an investigation was launched. Part of the investigation team

17 The investigation begins Having detected the binary deficiency, an investigation was launched. The first action was to look for evidence in an independent sample. There is another study of metal poor stars selected by their high-proper motion (Ryan 1989, Ryan & Norris 1991). We selected 472 stars from this sample ([Fe/H] <-0.4, δ >-25 o ), making independent radial velocity measurements. After eliminating uncertain cases, we were left with 349 isolated stars and 63 binaries. Ryan sample (1989)

18 The investigation begins Having detected the binary deficiency, an investigation was launched. The first action was to look for evidence in an independent sample. There is another study of metal poor stars selected by their high-proper motion (Ryan 1989, Ryan & Norris 1991). We selected 472 stars from this sample ([Fe/H] <-0.4, δ >-25 o ), making independent radial velocity measurements. After eliminating uncertain cases, we were left with 349 isolated stars and 63 binaries. Ryan sample (1989) The same binary deficiency was found!

19 Analysis of the evidence

20 Analysis of the evidence Retrograde Prograde Our first test was to plot the binary fraction as a function of V velocity. Binary fraction vs V velocity in km/s. We show results for the two studied samples and both combined. We only consider low metallicity stars ([Fe/H] < -1).

21 Analysis of the evidence Our first test was to plot the binary fraction as a function of V velocity. The binary deficiency for V<-300 km/s was immediately apparent. Binary fraction vs V velocity in km/s. We show results for the two studied samples and both combined. We only consider low metallicity stars ([Fe/H] < -1).

22 Analysis of the evidence Our first test was to plot the binary fraction as a function of V velocity. The binary deficiency for V<-300 km/s was immediately apparent. This conclusion was confirmed by the statistics. Velocity range Number of stars (combined) f (CLLA) Mean binary fractions f (Ryan) f (combined) V > -220 km/s ± 4 21 ± 5 28 ± 3 V < -300 km/s ± 3 12 ± 4 10 ± 3

23 Analysis of the evidence Our first test was to plot the binary fraction as a function of V velocity. The binary deficiency for V<-300 km/s was immediately apparent. This conclusion was confirmed by the statistics. A Kolmogorov-Smirnov test indicated that we could reject the null hypothesis, that isolated and binary stars come from the same parent population, with a % confidence level. Cumulative distribution function for isolated stars and binaries as a function of V velocity

24 Analysis of the evidence We next investigated whether the deficiency showed up as a function of the other velocity components, or the metallicity, too. U W [m/h] In all cases the result was negative

25 The possible scenarios

26 The possible scenarios We identify two possible scenarios to explain the lack of binaries: A selective destruction of binaries in retrograde orbits, An additional source of isolated stars in those orbits.

27 The possible scenarios We identify two possible scenarios to explain the lack of binaries: A selective destruction of binaries in retrograde orbits, An additional source of isolated stars in those orbits. The simplest case is the selective destruction of binaries. However, since we have 170 stars with V < -300 km/s, the binary fraction for the full sample (25%) means that there ought to be ~40 binaries in this range, but we only observe 17. This implies that a bit over 50% of the binaries initially in this velocity range have been destroyed! This is a very large effect which is difficult to explain. 25% Global Retrograde

28 The possible scenarios We identify two possible scenarios to explain the lack of binaries: A selective destruction of binaries in retrograde orbits, An additional source of isolated stars in those orbits. The simplest case is the selective destruction of binaries. However, since we have 170 stars with V < -300 km/s, the binary fraction for the full sample (25%) means that there ought to be ~40 binaries in this range, but we only observe 17. This implies that a bit over 50% of the binaries initially in this velocity range have been destroyed! This is a very large effect which is difficult to explain. The second option is even harder to sustain: To observe only 17 binaries with V < -300 km/s, there ought to be 68 stars, not the 170 seen. So, we need to add more than 100 stars to dilute the binary fraction down to the observed value. This represents an addition of ~150% more stars! 25% 25% Global Retrograde

29 The possible scenarios We identify two possible scenarios to explain the lack of binaries: A selective destruction of binaries in retrograde orbits, An additional source of isolated stars in those orbits. Since the magnitude of the required effect is smaller in the case of selective destruction (removal of 50% of binaries vs adding 150% of isolated stars), we decided to investigate first destruction mechanisms.

30 The lineup of suspects

31 The lineup of suspects To destroy binaries, we identify the following possibilities: Encounters with massive objects Tidal effects.

32 Encounters with massive objects Can encounters with massive objects selectively destroy binaries in retrograde orbits? For a gravitational encounter to be destructive, it must be impulsive, this means that it must happen in a time shorter than the binary period. This is analogous to a car passing a speed bump: if the perturbation time (passing the speed bump) is shorter than the response time of the shock absorbers, the effect is large: don t drive fast past speed bumps! If we drive over the same bumps slowly, we are in the adiabatic regime, where the car is not damaged.

33 Encounters with massive objects Can encounters with massive objects selectively destroy binaries in retrograde orbits? A binary in a retrograde orbit encounters other objects in the Galaxy at a larger relative velocity, and so it could be in the impulsive regime, while binaries in prograde orbits are not. Perhaps this is the effect we are looking for

34 Encounters with massive objects However, a binary is a tough object that is difficult to break. For an encounter to be impulsive, the collision time (t col ) must be shorter than the binary period (P): t col = (2b / v) < P b is the impact parameter and v the relative velocity. In units appropriate for this problem, this can be written as: (b /UA) < 10.5 (P / years) (v /10 2 km / s) Even considering the widest, most fragile binaries in our sample (P~10 years) and very fast encounters (v~400 km/s), the impact parameter must be unrealistically small (b~0.002 pc!).

35 Encounters with massive objects Making a more elaborate calculation, taking into account adiabatic corrections, does not change the previous conclusion. The two diagonal lines in the figure correspond to encounters where the energy injected to the binary by the collision equals its initial binding energy. Sample The abscissa is the binary semimajor axis, the ordinate is the collision impact parameter. We have assumed a point mass perturber of mass 10 6 M O. The encounter relative velocity is shown next to each line. Points below and to the right of the respective diagonal are disruptive encounters.

36 Encounters with massive objects Leaving theoretical prejudices aside, we looked for direct observational evidence of binary destruction due to objects within the galactic disk, or the central region of the Galaxy. W VW! (V + V circ ) 2 + W 2 However, no effect is apparent as a function of W, or VW at low velocities.

37 Tidal effects Any external perturbation will affect mainly widely separated binaries, since these are the least bound. However, when we compare the periods of binaries in prograde (V >-200 km/s) and retrograde (V <-300 km/s) orbits, we don't see less retrograde motion binaries with long periods. Retrograde Prograde

38 The lineup of suspects We conclude that there is no destruction mechanism that could selectively destroy binaries in retrograde orbits and produce the observed deficiency. We are forced to consider the alternative: mechanisms that could have added isolated stars in retrograde orbits, although the magnitude of the necessary effect is large.

39 The lineup of suspects In the case of addition of isolated stars, we consider: Break up of binaries and subsequent ejection of isolated stars from globular clusters due to star-binary encounters, In this case, we rely on the high density cluster environment to promote starbinary encounters that split binaries and eject single stars out of the cluster.

40 The lineup of suspects In the case of addition of isolated stars, we consider: Break up of binaries and subsequent ejection of isolated stars from globular clusters due to star-binary encounters, dissolution of globular clusters with preferential population of single stars in their outermost layers. In this case, energy exchange during an encounter promotes energy equipartition, sinking the heavy binaries while kicking isolated stars to the evaporating cluster envelope.

41 Binary break up Halo globular clusters have a mean V velocity of ~ -200 km/s, with a dispersion of ~100 km/s. V Retrograde Prograde LSR 0

42 Binary break up Halo globular clusters have a mean V velocity of ~ -200 km/s, with a dispersion of ~100 km/s. If binaries in this clusters are destroyed and its members expelled with a speed of ~100 km/s, this will produce an excess of isolated stars with a V velocity between ~-300 and ~-100 km/s Retrograde Prograde LSR 0 V

43 Binary break up Halo globular clusters have a mean V velocity of ~ -200 km/s, with a dispersion of ~100 km/s. If binaries in this clusters are destroyed and its members expelled with a speed of ~100 km/s, this will produce an excess of isolated stars with a V velocity between ~-300 and ~-100 km/s. Since the samples we have studied come from proper motion catalogues, we would not see the excess at V~-100 km/s, as clearly as the excess kicked in retrograde motion. Furthermore, the prograde motion excess would be diluted by the disk population. Selection function V Retrograde Prograde LSR 0

44 Binary break up However, there are several difficulties with this scenario: To have orbital velocities of ~100 km/s, binaries should have periods of a few days: m 1 +m 2 = 2 M O and v orb ~100 km/s P~20 days. It is very unlikely that there were enough short period binaries to explain the observed effect.

45 Binary break up However, there are several difficulties with this scenario: To have orbital velocities of ~100 km/s, binaries should have periods of a few days: m 1 +m 2 = 2 M O and v orb ~100 km/s P~20 days. It is very unlikely that there were enough short period binaries to explain the observed effect. Besides, the binary break up would produce an isotropic ejection and some effect ought to be apparent on the other velocity components, where, as we have seen, there is nothing anomalous.

46 Globular cluster dissolution There are also several difficulties with this hypothesis:

47 The complete story For those interested in reading the whole story, the reference is: A Survey of Proper Motion Stars. XVII. A Deficiency of Binary Stars on Retrograde Galactic Orbits and the Possibility that ω Centauri is Related to the Effect. B. Carney, L. Aguilar, D. Latham & J. Laird (2005) AJ 129,

48 The final report We have discovered a statistical significative deficiency in the number of binary stars in retrograde galactic orbits in a local sample of lowmetallicity, high-proper motion stars. The deficiency does not depend on the metallicity, or the U and W velocity components. Exploramos varias posibilidades para explicar esta deficiencia, desde una destrucción selectiva de binarias, hasta una fuente preferencial de estrellas solitarias, en órbitas retrógradas. La única posible explicación que encontramos es que nuestra muestra este contaminada con los restos de una galaxia enana esferoidal.