White Dwarfs

Mass ~ 1 M☉

Radius ~ 1 R🜨

Radius ~ 1 R🜨

ESA/NASA

Compact Objects

Mass ~ 1 M☉

Radius ~ 1 R🜨

Mass ~ 1 M☉

Radius ~ 10 Km

White Dwarf

Neutron Star

ESA/NASA

Life Cycles of Stars

• White Dwarfs:

  • The fate of most stars

  • Stars < 8 M☉

• Lower Left HR diagram
  • Blue and Faint

ESO

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

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)

Cataclysmic Variables

R. Hynes

X-ray

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

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

Color-Magnitude Diagram

Lugger et. al  (2017)

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

NGC 6752

Bright CVs

Faint CVs

Finding CVs in X-rays

• Advantage of X-ray Searches:

  • Avoid crowding

• Disadvantage:

  • L_x ≥ 10²⁹ ergs/s

What do we know from theory and simulations?

• Most L_x < 10²⁹ ergs/s
• > 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

Summary CVs in GCs

• Few of CVs per cluster

  • ~ 100s predicted

• X-ray biased sample:

  • L_x > 10²⁹ergs/s

• Period distribution?

  • 16 known periods

All Magnetics ?

• 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

• Focus: Cataclysmic Variables (CVs) with HST
• First Survey for CVs in GCs Unbiased in the X-ray
  • 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

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

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

Zhao et al. , including Pichardo Marcano (2020)

A new candidate Redback MSP

Pichardo Marcano et. al (2021a)

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

Blue Variables

Light Curves of Known CV Candidates

High and Low state?

Nova-like Candidate

Pichardo Marcano et. al (in prep)

U19

U21

Nova-like Candidate

First Magnetic He WD Candidate

Pichardo Marcano et. al (in prep)

Hα Excess

Hα deficiency

Magnetic He WD

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

  • 18.4 hours 

Pichardo Marcano et. al (in prep)

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

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?

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

AM Canum Venaticorum AM CVn

• White dwarf + White dwarf

• White dwarf + He Star

• Orbital Period: 5-70 minutes

Caltech

Caltech

• Porb 20 to ~ 60 minutes (?)

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

  • SO: Superhumps (periodic brightness variation )

• Disk instability model (DIM)

  •  Constant Mass transfer rate

  • Tidal-thermal disk instability model

Outbursting Systems

Empirical Relationships

1. T_rec ∝ P_orb^7.35
2. T_dur ∝ P_orb^4.54
3. Δ mag ∝ P_orb
4. Ground-Based:
   1. PTF/CRTS
   2. ZTF
   3. LINEAR

Levitan et al. 2015

See also Duffy et al. 2021

Empirical Relationships

1. T_rec ∝ P_orb^7.35
2. T_dur ∝ P_orb^4.54
3. Δ mag ∝ P_orb
4. Ground-Based:
   1. PTF/CRTS
   2. ZTF
   3. LINEAR

Levitan et al. 2015

See also Duffy et al. 2021

Better LC → Better Constraints

Transiting Exoplanet Survey Satellite (TESS)

Outbursts

Pichardo Marcano et al. (2021)

• Precursor
  • TESS (Duffy et al. 2021)
• Echo Outbursts
• DIM with Enhanced Mass Transfer  (e.g. Hameury & Lasota 2021)

TESS AM CVn Outbursts Survey

Pichardo Marcano et al. (2021)

ZTF

TESS

Long-term monitoring

Pichardo Marcano et al. (2021)

• Orbital Periods:
  • 2-minute cadence
• Limits on Recurrence Time
• "Normal Outburts"

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
• Color Evolution

Other work with Accreting WDs

• Follow-up eROSITA transient:

• TESS time series analysis

• • Atel: Pichardo Marcano (2020)

  • Paper: Schwope, [...], Pichardo Marcano, et al. (2021)

Caltech

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

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)

Future Work

NGC 6397

M 92

Spectroscopy (MUSE)

AO Gemini IR

Blue Straggler

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

New Variables in the Core

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

Extra

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

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

• Most L_x < 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

Evolution of the CV

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

Knigge (2006)

PTF1 J0719+4858:

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

Color Evolution

Pichardo Marcano et al. (2021)

Ramsay Gaia

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)

AM CVns

•  Helium Rich and Know to go into outburts

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