A search for variable stars and compact binaries in globular clusters with HST

Manuel Pichardo Marcano

Liliana Rivera Sandoval, Thomas J. Maccarone

Astronomy group meeting April, 2020

Cataclysmic Variable

  • White dwarf primary

  • "Main-sequence" donor

  • Roche-lobe overflow

  • Accretion disk (non-magnetic)

  • Disk instability:

    • Dwarf novae

    • 2-8 mag outburst

Formation of CVs

Ivanova et. al (2012)

  1. Main sequence binary
  2. Roche lobe overflow
  3. Common Envelope
    • α = Ebin/ΔEorbit
  4. Detached binary
  5. Cataclysmic Variable

Evolution of the CV

  • Magnetic Breaking
    • Porb > 3 hr
  • Period Gap
    • Fully convective donor
  • Gravitational Wave Rad
    •  Porb < 2 hr
  • Period Minimum
    • Brown dwarf
  • Period Bouncers
    • Increase Porb

Knigge (2006)

Evolution of the CV

  • Tracer evolution
  • Donor mass
  • Ṁ

Abril et. al (2019)

CV Population in the field

Pala et. al (2019)

  • Dwarf Novae (59 %):
    • U Gem
    • SU UMa
    • WZ Sge

 Mroz et. al (2016) Image: (J. Skowron, K. Ulaczyk / Warsaw University Observatory)

Variability Search

Variability Search

My God, It's Full of Stars

NASA, ESA, H. Richer and J. Heyl (University of British Columbia), and J. Anderson and J. Kalirai (STScI)

Binaries in Globular Clusters

  • Globular Clusters
    • The binary population drives the dynamical evolution of GCs
  • Cataclysmic Variables:
    • Large (predicted) sample at known distance
    • Potentially very different from field CVs
    • A lot of open questions
  • Gravitational Waves:.
    • White dwarf degenerate sources for LISA

Looking of CVs in GCs

NASA, ESA, H. Richer and J. Heyl (University of British Columbia), and J. Anderson and J. Kalirai (STScI)

Variability Search in GCs

The results of our extensive survey provide new evidence ... that ordinary DNe are indeed very rare in GCs.

Pietrukowicz et. al (2008)

Hourihane et. al (2011)

  • Only 17 confirmed DNe
  • 7 GCs

Modiano et. al (2020)

Finding CVs in GCs

HST Optical (3' x 2')

NASA, T. Brown and S. Casertano (STScI)

HST NUV (2.7' x 2.7')

Dieball et. al (2016)

Chandra (3.65 ' x 3.65')

NASA/CXC/Northwestern Univ/J.Fregeau

1. X-rays and 2. (F)UV excess

Color-Magnitude Diagram

47 Tuc

L.E. Rivera Sandoval et al.  (2018)

  • CMDs:
    • Identify blue objects
    • Hα excess
  • Variability:
    • Flickering
    • Ellipsoidal Modulations
    • Outbursts

Color-Magnitude Diagram

NGC 6752

Lugger et. al  (2017)

  • CMDs:
    • Identify blue objects
    • Hα excess
  • Variability:
    • Flickering
    • Ellipsoidal Modulations
    • Outbursts

Finding CVs in GCs

  • Advantages:

    • Avoid crowding

  • Disadvantages:

    • Small FoV (FUV)
    • Lx ≥ 1029 ergs/s

Significant X-ray bias

X-ray: NASA/CXC/CfA/J.Grindlay & C.Heinke; Optical: ESO/Danish 1.54-m/W.Keel et al.

Location, Location, Location

CVs in Globular Clusters

47 Tuc

NGC 6752

NASA, ESA, H. Richer and J. Heyl (University of British Columbia), and J. Anderson and J. Kalirai (STScI)

ESA/Hubble, NASA

Core-collapse vs non-core collapsed

CVs in Globular Clusters

Belloni & Rivera Sandoval  (2020)

Location, Location, Location

47 Tuc

L.E. Rivera Sandoval et al.  (2018)

CVs in Globular Clusters

43 CVs

16 CVs

NGC 6752

Lugger et. al  (2017)

Belloni & Rivera Sandoval  (2020)

CVs in Globular Clusters

Belloni & Rivera Sandoval  (2020)

Radial distribution

CVs in Globular Clusters

CVs in GCs

  • Few of CVs per cluster

    • Tens per cluster at most

  • X-ray biased sample:

    • Lx > 1029ergs/s

  • Dearth of DNe:

    • Only 17 confirmed

  • Core-collapsed:

    • Bimodal population

  • Non core-collapsed:

    • 1 faint population

  • Period distribution?

    • Only 16 know periods

GC # CV candidates
47 Tuc 43
Omega Cen 27
NGC 6397 (CC) 15
NGC 6752 (CC) 16
... ...

MOCCA Models

Belloni et. al (2019)

  • ∼100-1000 CVs per Cluster
  • # scales with cluster mass
  • α dependence
    • Common envelope phase efficiency

 

  • Dearth of dwarf novae:
    • Small duty cycle (< 6%)
  • > 90% are period bouncers
    • Old systems
  • Most Lx < 1029 ergs/s
  • Bright CVs dynamically formed
  • Population beyond half-light radius

Simulation results

Belloni et. al (2019)

Need more detections. Need more Periods

Unbiased X-ray search

7 Globular Clusters in the core and outskirts

Core vs Outskirts

NGC 6397

  • CMDs:
    • Identify blue objects
  • Variability:
    • Flickering
    • Ellipsoidal Modulations
    • Outbursts
  • Ground follow-up

Other Binaries

  • Low-mass X-ray Binary
  • Millisecond Pulsars
  • Active Binaries

Chandra and Radio follow-up

NASA/CXC/Northwestern Univ/J.Fregeau

  • Chandra X-ray
  • MAVERIC Survey

Chandra and Radio follow-up

Zhao et. al (2020)

Preliminary Results

NGC 6397

  • WFPC-2
  • March-April 2005:
    • 126 orbits
    • F814W, F606W and F336W
    • Exp time: 500-700 s

Preliminary Results

NGC 6397

  • WFPC-2
  • March-April 2005:
    • 126 orbits
    • F814W, F606W and F336W
    • Exp time: 500-700 s

A new candidate Redback MSP

  • Porb of 1.96 day

Zhao et. al (2020)

¡Gracias!

HST Variability in GCs

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HST Variability in GCs

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