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

Manuel Pichardo Marcano

Liliana Rivera Sandoval, Thomas J. Maccarone,

Arash Bahramian, Diogo Belloni, Craig Heinke

White Dwarfs

Mass ~ 1 M☉

Radius ~ 1 R🜨

Radius ~ 1 R🜨

Compact Objects

Mass ~ 1 M☉

Radius ~ 1 R🜨

Mass ~ 1 M☉

Radius ~ 10 Km

White Dwarf

Neutron Star

Stellar Remants

NASA

Life Cycles of Stars

• White Dwarfs:

  • The fate of most stars

  • Stars < 8 M☉

• Lower Left HR diagram
  • Blue and Faint

Cataclysmic Variables

• White dwarf primary

• "Main-sequence" donor

• Roche-lobe overflow

• Accretion disk (non-magnetic)

• X-ray sources

• Highly Variables:

  • Dwarf novae

    • 2-8 mag outburst

    • 10-100 increase in L

Cataclysmic Variables

• Magnetic CVs:
  • High B Field (Polars)
  • Weaker B Field  (IPs)
• Non-Magnetic:
  • Accretion disk
    • DN Outburst

Intermediate Polar (IPs)

Polars

"Disk" CVs

Cataclysmic Variables

• Highly Variable and Hydrogen Rich

JD - 2458636

Hα

DN

Hα:

3-2 H transition

Cataclysmic Variables

• Highly Variable and Hydrogen Rich
• Many types of Behaviour

High and Low States

Super Outburts

Formation of CVs

1. Main sequence binary
2. Roche lobe overflow
3. Common Envelope
4. Detached binary
5. Cataclysmic Variable

Evolution of the CV

• Magnetic Braking
  • 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

Abril et. al (2019)

CV Population in the field

Pala et. al (2019)

• Dwarf Novae (59 %):
  • Transients/Outbursts:
    • U Gem
    • SU UMa
    • WZ Sge
• Novalike:
  • Persistently Bright

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

Variability Search

Variability Search

Variability Search

TOROS: Transient Optical Robotic Observatory of the South

My God, It's Full of Stars

NGC 3766

NGC 6752

TOROS ‘First Light’

Images by Dario Grana, Horacio Rodriguez, and Dr. Matias Schneitter, IATE UNC. Images processed by Dr. Lucas Macri, TAMU

Globular Cluster

Open Clusters

Omega Centauri

Hubble Space Telescope

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

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)

Variability Search in GCs

[...] this would suggest MCVs are somehow enhanced in globular cluster cores, providing yet more evidence that stellar evolution in globulars is affected by close encounters.

Grindlay (1999)

Hourihane et. al (2011)

• DNe is suppressed due to truncation of the accretion disk.

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

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

Finding CVs in GCs

Color-Magnitude Diagram

Lugger et. al  (2017)

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

NGC 6752

Bright CVs

Faint CVs

Color-Magnitude Diagram

47 Tuc

L.E. Rivera Sandoval et al.  (2018)

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

Location, Location, Location

CVs in Globular Clusters

47 Tuc

NGC 6752

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

What do we know from theory and simulations?

• Most Lx < 10²⁹ ergs/s
  • ~100 L☉ of the Sun
• > 90% are period bouncers
  • Old systems
• Dearth of dwarf novae:
  • Low Accretors -> Rare Outburts
• Population beyond half-light radius

Simulation results

Belloni et al. (2016, 2017a, 2017b)

Simulation results

Finding CVs in X-rays and UV

• Advantages of X-ray Searches:

  • Avoid crowding

• Disadvantages:

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

Summary 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 known periods

All Magnetics ?

Accreting WD Surveys

1- SSCOVaS: Survey for Studying Compact Objects and Variables

2- TACOS: TESS AM~CVn Outbursts Survey

Pichardo Marcano et al. (2021b)

X-ray unbiased search

SSCOVaS: Survey for Studying Compact Objects and Variables Stars

• 7 Globular Clusters
• Many Exposures
• Different properties
  • Metallicity
  • Core-collapsed
  • Non core-collapse

SSCOVaS: Survey for Studying Compact Objects and Variables Stars

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

X-ray unbiased search

Other Binaries

Chandra and Radio follow-up

Credits: NASA/Rivera Sandoval

Main et al 2018 (Nature)

Redback 'spider' Pulsar

Redback  Pulsar

Preliminary Results

NGC 6397

• Closest Core Collapse
• Second Closest
  • 2.4 kpc
  • 15 CV candidates
  • 2 MSPs

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 Pulsar

HUGS catalogue. (Piotto et al. 2015, Nardiello et al. 2018)

Pichardo Marcano et. al (2021a)

X-ray

HST F336W

Radio

A new candidate Redback MSP

Pichardo Marcano et. al (2021a)

• Porb 1.96 days
• Longest Porb for Redback in GCs
• Missed MSP in Pulsar Searchers

Light Curves Known CV Candidates

High and Low state?

Novalike?

New Eclipser?

Light Curves Known CV Candidates

New CVCandidate

New CV Candidate

• New CV Candidate
• Not Detected X-rays
• Potential Porb

  • 5.21 hours 

NGC 6397

• We were able to recover all known CV Candidates
  • Light Curves
• Found Orbital Period Redback Pulsar
• Potential New, X-ray faint, CV Candidate
• Many more variables to analyze...

Preliminary Results

M 92

• Non-Core Collapse
• Very Metal-Poor
  • 8.4 kpc
  • 4-5 CV candidates
  • 1 Pulsar

Preliminary Results

M 92

• WFPC-2
• 2008-01-25 to 2008-01-30:
  • 76 Exposures
  • F555W
  • Exp time: 10 s

New Variables in the Core

HST vs Ground-based

Yepez et al. (2020)

Ground-based Data

HST Data

New Variables in the Core

• Pulsating Stars:
  • RR Lyrae
  • SX Phe
• Active Binaries
• Transients?
  • Dwarf Novae?

New Variables in the Core

SX Phe

• Good Distance indicator:
  • Strong P-L relation
• Less common than RR Lyrae
• Blue Straggler
  • Product of Binary evolution

P = 1.38 hr

Blue Straggler

Subramaniam et. al (2020) (Image credits: Snehalata Sahu)

New Variables in the Core

RR Lyrae

• Very common Pulsators
• Metallicity indicators

M 92

• New variables in the core:
  • SX Phe, RR Lyrae, AB, ....
• Search for Optical Counterparts of X-ray sources:
  • 1 Pulsar, 4-5 CVs
• Many more variables to analyze...

SSCOVaS: Survey for Studying Compact Objects and Variables

• First Unbiased Survey for CVs in GCs
• Optical Counterpart X-ray/Radio Sources in GCs
• Search for Orbital Periods :
  • New Redback Porb 1.9 days
• Spatial Distribution:
  • Looking at the dense Core of Clusters
  • Next: Search in the Outskirts

¡Gracias!

CVs in GCs

• Few of CVs per cluster

• X-ray biased sample:

  • L_x > 10²⁹ergs/s

• Dearth of DNe:

  • Only 17 confirmed

• Core-collapsed:

  • Bimodal population

• Non core-collapsed:

  • 1 faint population

• Period distribution?

  • 16 know periods

Cataclysmic Variable

• Magnetic and No

JD - 2458636

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

• 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)

A new candidate Redback MSP

• Porb of 1.96 day

Zhao et. al (2020)

• Chandra X-ray
• MAVERIC Survey

Chandra and Radio follow-up

Zhao et. al (2020)

Unbiased X-ray search

7 Globular Clusters in the core and outskirts