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

A needle in a haystack:

Finding Exotic Binaries in Old Star Clusters

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🜨

White Dwarfs

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)

• Highly Variables:

  • Dwarf novae

    • 2-8 mag outburst

    • 10-100 increase in L

Cataclysmic Variables

R. Hynes

X-ray

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

JD - 2458636

DN

One magnitude = 2.512 x brighter

Cataclysmic Variables

• Highly Variable and Hydrogen Rich
• Many types of Behaviour

High and Low States

Super Outburts

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

My God, It's Full of Stars

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

Globular Cluster

• 10⁵ - 10⁶ Stars
• Old systems:
  • 10 -13 Gyr
• ~150 Galactic GCs

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 Binaries Surveys

1- SCOVaS: Survey for Compact Objects and Variables

Pichardo Marcano et al. (2021a)

Pichardo Marcano et al. (in prep.)

2- TACOS: TESS AM CVn Outbursts Survey

Pichardo Marcano et al. (2021b)

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

SCOVaS: Survey for Compact Objects and Variables

SCOVaS: Survey for Studying Compact Objects and Variables

• First Survey for CVs in GCs Unbiased in the X-ray
  • CMD Position and Periodicity/Variability Searches
  • X-ray faint CVs
  • Orbital Periods
• Spatial Distribution:
  • Looking at the dense Core of Clusters
  • Next: Search in the Outskirts
• Optical Counterpart X-ray/Radio Sources in GCs

Preliminary Results

NGC 6397

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

Preliminary Results

NGC 6397

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

Preliminary Results

NGC 6397

We haven't found new

X-ray faint CVs

Spoiler Alert!

Preliminary Results

NGC 6397

We haven't found new

X-ray faint CVs

Spoiler Alert!

Other Binaries

Credits: NASA/Rivera Sandoval

Preliminary Results

NGC 6397

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

Other Binaries

Credit: Roen Kelly/Discover 

Main et al 2018 (Nature)

Redback 'spider' Pulsar

Redback  Pulsar

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 Pulsar

Pichardo Marcano et. al (2021a)

X-ray

HST F336W

Radio

A new candidate Redback Pulsar

Variable

Periodic

Lomb-Scargle periodogram

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

Blue Variables

Extremely Low-mass White Dwarf

Pichardo Marcano et. al (in prep)

Highly Variable

Hα Excess

Hα deficiency

Highly Variable

Pichardo Marcano et. al (in prep)

Hα Excess

Hα deficiency

Magnetic low-mass White Dwarf

• V_rad > 100 km/s (MUSE)
• Not Detected in X-rays
• Rotation Period

  • 18.4 hours 

Pichardo Marcano et. al (in prep)

ESO/L. Calçada

Pre-Cataclysmic Variable

Pichardo Marcano et. al (in prep)

Hα Excess

Hα deficiency

Detached White Dwarf-Red Dwarf

• Not Detected in X-rays
• Potential P_orb

  • 5.21 hours 

Pichardo Marcano et. al (in prep)

Detached White Dwarf-Red Dwarf

Light Curves of Known CV Candidates

High and Low state?

Nova-like Candidate

Pichardo Marcano et. al (in prep)

U19

U21

Nova-like Candidate

NGC 6397

• Pipeline to find interesting objects:
  • CMD Position and Periodicity/Variability Searches
• We were able to recover all known CV Candidates
• Found Orbital Period Redback Pulsar
• Detached White Dwarf-Red Dwarf
• First Magnetic He Core WD in GCs
• Many more variables to analyze...

Where are all the CVs?

SCOVaS: Survey for 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

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

Pichardo Marcano et. al (in prep)

HST LC

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...

Compact Binaries Surveys

1- SCOVaS: Survey for Compact Objects and Variables

Pichardo Marcano et al. (2021a)

Pichardo Marcano et al. (in prep.)

2- TACOS: TESS AM CVn Outbursts Survey

Pichardo Marcano et al. (2021b)

Compact Binaries Surveys

Accreting White Dwarf Binaries

Cataclysmic Variables

AM CVn

• Main Sequence Star
• P_orb ~ 80 min - 10 hours
• Outbursts:
  • 2-20 days
  • Disk instability model+ (DIM)
• >1000 in the galactic field
• Predicted large # in old Star Clusters

• White Dwarf/Helium Star
• P_orb ~ 5 - 70 min
• Outburts:
  • weeks?
  • DIM?
• ~70 in the galactic field

Mark A. Garlick

Caltech

Transiting Exoplanet Survey Satellite TESS

TESS Cadence and Coverage

• Primary mission:
  • 2-min selected targets
  • 30-min full-frame images (FFI).
• Extended:
  • 20 sec selected targets
  • 10-min FFI
• Second extended mission
  • 200-second FFI

AM Canum Venaticorum AM CVn

• White dwarf + White dwarf

• White dwarf + He Star

• Orbital Period: 5-70 minutes

Caltech

Disk AM CVns

High state (high Ṁ)

• Hot ionized disk

~20

~10

~40-60 (?)

Low state (low Ṁ)

• Cold stable disk

Caltech

Caltech

• P_orb 20 to ~ 60 minutes (?)

• Superoutburts (SO) and "Normal" outbursts (NO)

• Disk instability model (DIM)

  •  Constant Mass transfer rate

  • Tidal-thermal disk instability model

    • SO: Superhumps (periodic brightness variation )

Outbursting Systems

Tidal radius

3:1 Resonance radius

Accretion disk

Precessing eccentric disc

Kley et al. 2008

Kimura 2020

Caltech

• P_orb 20 to ~ 60 minutes (?)

• Superoutburts (SO) and "Normal" outbursts (NO)

• Disk instability model (DIM)

  •  Constant Mass transfer rate

  • Tidal-thermal disk instability model

    • SO: Superhumps (periodic brightness variation )

Outbursting Systems

Better LC → Better Constraints

Place Constraints

Tidal radius

3:1 Resonance radius

Accretion disk

1. T_rec ∝ P_orb^7.35
2. T_dur ∝ P_orb^4.45
3. Δ mag ∝ Porb
4. Ground-Based:
   1. PTF/CRTS
   2. LINEAR
   3. ZTF

Levitan et al. 2015

See also Duffy et al. 2021

Better LC → Better Constraints

Constraint Empirical Relationships

Outbursts

Pichardo Marcano et al. (2021)

SO duration

ASASSN-14cc

Pichardo Marcano et al. (2021)

9–18 d

kato et al. (2015)

• 2 Super Outbursts:
  • P_orb 22.5 min
  • Variable in length
    • Compare Kato+'15 and Duffy+'21
• Superhump period
  • 10 minute cadence

~7 d

• Precursor
  • TESS (Duffy et al. 2021)
• 5/6 have Precursor
  • Except ASASSN-14cc

PTF1 J0719+4858

Pichardo Marcano et al. (2021)

ZTF

TESS

Precursor

Type of Super Outbursts

Osaki et al. (2005)

High Mass Ratio

DIM

Cataclysmic Variables

See also Duffy et al. (2021)

High Mass Ratio

High Mass Ratio

High Mass Ratio

 eccentric disc develops rapidly

Type of Echo Outbursts

SDSS J1043+5632

• P_orb = 28.5 min
• WZ Sge like
  • Precursors
  • EMT (Hameury+'20)
  • Reflection:
    • Hot Massive WD

• P_orb = 26.7 min
• Compare to TCP J21040470+4631129
  • EMT (Hameury+'20)
• Helium "SU UMa"
  • EMT+TTI (Kotko+'12)

SO

rebrightening

SO

rebrightening

• Color Evolution

PTF1 J0719+4858

Pichardo Marcano et al. (2021)

ZTF

SO

NO

SO

NO

rebrightening

rebrightening

Color Evolution

PTF1 J0719+4858

SS Cyg

Cataclysmic Variable

• Similar to Dwarf Novae

  • Hameury et al. (2020)

• Similar SDSS J1411+4812
  • Rivera Sandoval et al. (2021)
• Compare to SDSS 1137
  • Rivera Sandoval et al. (2021)

Pichardo Marcano et al. (2021)

Color Evolution

PTF1 J0719+4858

• Similar to Dwarf Novae

  • Hameury et al. (2020)

• Similar SDSS J1411+4812
  • Rivera Sandoval et al. (2021)
• Compare to SDSS 1137
  • Rivera Sandoval et al. (2021)

Pichardo Marcano et al. (2021)

SDSS J113732+405458

 Enhanced mass transfer

Rivera Sandoval et al. (2021)

P_orb = 26.7 min

P_orb ~ 60 min

• Color Evolution

SDSS J1043+5632

Pichardo Marcano et al. (2021)

ZTF

TESS

SDSSJ1043+5632

• Similar to Dwarf Novae:
  • Hameury et al. (2020)
  • Rivera Sandoval et al. (2021)

Color Evolution

Long-term monitoring

• Observed 357 days
• Recurrence Time:
  • 203 days
• P_orb = 30.14 min :
  • 2-minute cadence
• "Normal Outburts"

Sector 5

Sector 12

Normal Outbursts

V803 Cen

• V803 Cen
  • P_orb = 26.9 min
  • "Cycle"
  • Low Ṁ
• PTF1 J2219+3135:
  • New P_orb= 27.7 min
  • Agrees with T_rec
  • Permanent Superhumper?

Superoutburst duration vs P_orb

Pichardo Marcano et al. (2021)

TACOS Summary

• Precursors and "Normal" Outbursts:
  • Relatively Common
  • Short Duration (missed by ground-based)
• Echo Outbursts:
  • Different morphology
  • Enhanced Mass Transfer
• Superoutburst durations overestimated
• Two new candidate orbital periods
  • Constraint on T_rec vs. P_orb
• Color Evolution

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)

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)

Unbiased X-ray search

7 Globular Clusters in the core and outskirts