binaries, blue stragglers, and other exotic objects in the cmd

Antonino P. Milone

Binaries, Blue Stragglers,and other exotic objects in the CMD

A. P. Milone Stellar Populations University of Padova, 2021

NIR picture of MW from emission of dust.

Differential reddeningReddening across the sky is not uniform


The RGB Bump

The RGB bump appears as a clump of stars along the RGB.It is well recognisable as an excess in the luminosity function of RGB stars (Iben 1968).

Bono et al.(2010)

47 Tucanae


The red clump– The red clump (RC) is a clustering of red giants in the CMD.

– RC stars are bluer (hotter) than RGB stars with the same luminosity (Teff~5000K).

– RC stars and horizontal branch stars are burning helium in their cores.

In the dichotomy between the RC and the HB, RC stars are associated withyounger and more metal-rich populations than those associated with the HB. (See review by Girardi 2016).

Milone (2015)


The ESO near-infrared survey (VVV) exploits the VistaTelescope to cover ~520 square degrees

Photometric survey the Galactic Bulge

Hess CMD of 66 million stars from 300 square degree in the Bulge. Minniti et al.(2014).


The color and magnitude of red-clump stars can changedramatically in CMDs of stars in different fields of view.

Red Clump and reddening towards the Bulge

Nataf et al.(2015)


Nataf et al.(2013)

RGB Bump and reddening towards the Bulge

The relative color and magnitudes of the red clump are indicative of the amount of differential reddening.Moreover, they constrain the reddening direction, hencethe reddening law.

ONE-MILLION $ ASSIGNMENTSimilarly to Pal 2, some stellar systems exhibits broadened sequences in their CMDs.

Big question:Is it a signature of multiplepopulations with differentmetallicities? Different ages?

Is it due to differential reddening?Is it due to both differential reddening and multiple populations

Define a method to correct the CMD for differential reddening (you can use the file with photometry on moodle).

Antonino P. Milone

Binaries, Blue Stragglers,and other exotic objects in the CMD


Star clusters are described in terms of:

– Core radius (rc): the distance at which the surface luminosity is dropped by a factor of 2

– Half-light (mass) radius (rh): the radius within which half of the luminosity (mass) is included.

– Tidal radius (rt): the radius where the gravitational influence of the galaxy on cluster stars is larger than the influence of the cluster.

Luminosity profiles of star clusters


King profile


We can define the concentration of the model:

c=log10(rt/rc)

Collection of King profiles for various values of c [0.5–2.5]. From King (1966).

Ivan R. King


Zocchi et al. (2012)


Core collapse

Projected luminosity profile of the core-collapsed globular cluster M30. Note the steep cusp in the inner region.


In ~15% of GCs the luminosity continues to increase steadily all the way to the core region.

The core collapse is a catastrophic dynamical process consisting in the runaway contraction of the core of a star cluster

Binary-binary and binary-single collisions are thought to halt (or delay) the collapse of the core, thus avoiding infinite central densities.

Intermediate-mass black holes

Velocity-dispersion profile of the massive GC NGC6388. Gray triangles are from Lutzgendorf et al. (2011), Black squares and circles from Lanzoni et al. (2013). Red lines are Jeans models corresponding to different masses of

the central BH.


– Core collapse is not the only responsible for the luminosity increase.

– An intermediate-mass or massive black hole (BH) at core can also produce a cusp.

– The presence of black holes in the centers of globular clusters is an open issue of present-day astrophysics.

If we consider the two components of an unresolved binary system and indicate with m1, m2, F1, and F2 their magnitudes and fluxes.

The binary system will appear as a single point-like source with magnitude:

Indeed:

m1=-2.5 log(F1)m2=-2.5 log(F2)

mbin=-2.5 log(F1+F2) =-2.5 log[F1 (1+F2/F1)] =-2.5 log(F1) -2.5 log(1+F2/F1) = m1-2.5 log(1+F2/F1)

Unresolved binariesUnresolved binaries

Equal-mass binaries are binary systems formed by two stars withthe same mass M1=M2.

In a simple stellar population: m1=m2, F1=F2

As a consequence: mbin= m1-2.5 log(1+F2/F1) = m1-2.5 log(1+1) = m1-2.5 log(2) = m1-0.752

The binary system will appear0.752 mag brighter than eachsingle star.


Milone et al. 2012Milone et al. 2012

As an example, the V and I magnitudes of two equal-mass binaries are:

Vbin=V1-0.752Ibin=I1-0.752

Their color is

Vbin-Ibin=(V1-0.752)-(I1-0.752)=V1-I1

The binary system composed of equal-mass stars has the same color as each single star.


Milone et al. 2012Milone et al. 2012

Binaries in star clusters


– Measuring the fraction of binaries in a large number of clusters:

– We can detect only binaries with large mass ratio (e.g. q>0.5).

In the ideal case:– region A of the CMD is populated by both single stars and binaries.

– Region B is populated by binaries with q>0.5.

– The fraction of binaries is provided by the ratio of number of stars in the region B and A.

In reality regions A and B are contaminated by: field stars (non cluster members), apparent binaries (blends+stars with large errors).



– Artificial stars are used to estimate the numbers of apparent binaries in the regions A and B.

– Proper motions and/or models of the Galactic field are used to estimate the numbers of field stars in the regions A and B

– Measuring the fraction of binaries in a large number of clusters:

Milone et al. (2012)



– The fraction of binaries anti-correlates with the mass of the host cluster. Massive clusters have less binaries than low-mass ones.

– The fraction of binaries does not depend neither on age nor on stellar collision rate.

Fra

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Absolute GC magnitude (Mv)

Milone et al. (2012)Possible conclusions:

Massive clusters are more efficient in destroying binaries

Collision is not an efficient channel in producing binaries



– The fraction of binaries, when properly normalized, follows a trend that is common to most clusters.

– It is maximum in the innermost cluster regions, drops by a factor of ~2, at a distance of two core radii and approach its minimum of ∼20 per cent at about five core radii.

No

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Log (R/Rcore)Milone et al. (2012, 2016)



– The fraction of binaries does not depend on the mass ratio.

Nor

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lized

bin

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Mass ratio (q)



– The fraction of binaries does not depend on the mass of the primary stars.

Nor

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Mass of the primary starMilone et al. (2012, 2016)



– The fraction of binaries depends from the mass of the host cluster

– The normalized fraction of binaries depends on the radius (normalized to the core radius).

Fra

ctio

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bin

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Absolute GC magnitude (Mv)

Milone et al. (2012, 2016)

Nor

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bin

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frac

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Log (R/Rcore)


– The binary fraction anti-correlates with mass of the host cluster.

Fra

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bin

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→A proxy for visible massAbsolute GC magnitude (Mv)

Milone et al. (2012, 2016)

Research on UFDsResearch on UFDs

– What is the mass of Horologium I?

Blue Stragglers

Globular clusters are prototypes of old simple stellar populations. Their stars share all the same age.

Blue Stragglers

Blue stragglers are main sequence stars in a stellar populations that are brighter and bluer than stars at the main sequence turn off.

Blue Stragglers

BSSs have been discovered by Allan Sandage in 1953, while working on the CMD of M3.

Blue stragglers seem consistent with stars that are younger than the bulk of stars.

Blue Stragglers

Blue stragglers seem consistent with stars that are younger than the bulk of stars.

Blue Stragglers

– Field stars trapped by the cluster?

– Recent star formation within the cluster?

– We can immediately exclude differential reddening!

binaries, blue stragglers, and other exotic objects in the cmd

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