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🜨

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

Magnitude

JD - 2458636

Szkody et. al (2020)

Hα

Hα:

3-2 H transition

Cataclysmic Variables

Highly Variable and Hydrogen Rich
Many types of Behaviour

Szkody et. al (2020)

High and Low States

Super Outburts

Formation of CVs

Ivanova et. al (2012)

Main sequence binary
Roche lobe overflow
Common Envelope
Detached binary
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)

Brighter

Bluer <---> Redder

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

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)

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)

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

Hα Excess

NGC 6752

Hα deficiency

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

Hα Excess

Hα deficiency

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)

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

Predicted "Hidden" CV Population

Finding CVs in X-rays and UV

Advantages of X-ray Searches:
- 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.

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

Need more detections. Need more 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

Zhao et al.(2020)

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

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

Need more detections. Need more Periods

Cataclysmic Variable

Magnetic and No

Magnitude

JD - 2458636

Szkody et. al (2020)

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

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

White Dwarfs

Compact Objects

Stellar Remants

Life Cycles of Stars

Cataclysmic Variables

Cataclysmic Variables

Cataclysmic Variables

Cataclysmic Variables

Formation of CVs

Evolution of the CV

Evolution of the CV

CV Population in the field

Variability Search

Variability Search

Variability Search

My God, It's Full of Stars

TOROS ‘First Light’

Hubble Space Telescope

Binaries in Globular Clusters

Looking of CVs in GCs

Variability Search in GCs

Variability Search in GCs

Finding CVs in GCs

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

Finding CVs in GCs

Color-Magnitude Diagram

Color-Magnitude Diagram

Location, Location, Location

CVs in Globular Clusters

Location, Location, Location

CVs in Globular Clusters

CVs in Globular Clusters

Radial distribution

CVs in Globular Clusters

What do we know from theory and simulations?

Simulation results

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

Simulation results

Predicted "Hidden" CV Population

Finding CVs in X-rays and UV

Advantages of X-ray Searches:

Disadvantages:

Significant X-ray bias

Summary CVs in GCs

Need more detections. Need more Periods

Accreting WD Surveys

1- SSCOVaS: Survey for Studying Compact Objects and Variables

2- TACOS: TESS AM~CVn Outbursts Survey

X-ray unbiased search

X-ray unbiased search

Other Binaries

Chandra and Radio follow-up

Redback 'spider' Pulsar

Redback Pulsar

Preliminary Results

NGC 6397

Preliminary Results

NGC 6397

Preliminary Results

NGC 6397

A new candidate Redback Pulsar

A new candidate Redback MSP

Light Curves Known CV Candidates

Light Curves Known CV Candidates

New CVCandidate

New CV Candidate

NGC 6397

Preliminary Results

M 92

Preliminary Results

M 92

New Variables in the Core

New Variables in the Core

New Variables in the Core

Blue Straggler

New Variables in the Core

M 92

SSCOVaS: Survey for Studying Compact Objects and Variables

¡Gracias!