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

Manuel Pichardo Marcano

Liliana Rivera Sandoval, Thomas J. Maccarone

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

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)

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

NGC 6752

Lugger et. al  (2017)

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

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

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)

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.

CVs in GCs

  • Few of CVs per cluster

  • 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?

    • 16 know periods

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

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!