Teresa Sicignano

1

25/05/2026 CosmoVerse@Sofia

Session 1

State of the Hubble Tension (L)

Distance Ladder: Gaia, eclipsing binaries, maser, SNIa, Hubble flow (L)

Short intro on Cepheids (HR diagram, kappa mechanism) (L)

Recent updates: clusters, JWST results (L)

Type II distance ladder: https://ui.adsabs.harvard.edu/abs/2025ApJ...987...87J/abstract

​

Session 2

More details on Cepheids: PL relation and why there is one (theory) (T)

Discovery of the PL relation by Henrietta Leavitt (Louise has slides)

Wesenheit definition (correct for dust)

Different classes of Cepheids (Type II, anomalous) (T)

Show light curves (T)

Population II distance ladder (see Mauricio’s paper) (T)

​

Session 3 

Cepheid metallicity dependence: different methods (L)

Metallicity measurements (T)

​

Session 4

Notebooks and exercises (T&L)

How to use Cepheids: applications -> notebook (T&L)

Notebook 1 : Gaia calibration in MW + include metallicity -> scatter decreases

Notebook 2 : application to measure distance to a galaxy

Notebook 3: measure distance to the LMC

https://github.com/lbreuval/Tutorials_Cosmoverse/tree/main

�

https://github.com/lbreuval/Tutorials_Cosmoverse/blob/main/CosmoVerse_Cepheids_Tutorial_1.ipynb

https://github.com/lbreuval/Tutorials_Cosmoverse/blob/main/CosmoVerse_Cepheids_Tutorial_2.ipynb

Teresa Sicignano

​

PhD student at ESO (Garching) - Scuola Superiore Meridionale - INAF – OACN (Naples)

CosmoVerse School@Sofia 2026

Measuring astronomical distances with Cepheid variables

Teresa Sicignano

3

25/05/2026 CosmoVerse@Sofia

Riess+2022

The extragalactic distance scale. The three steps to H₀

• Geometric indicators to calibrate the Period-Luminosity (PL) relations of Cepheids

​

• PL relations to calibrate the absolute magnitude of the peak luminosity of SN Ia

​

• SN Ia distance + recession velocity 🡪 H₀

1

2

3

Teresa Sicignano

4

25/05/2026 CosmoVerse@Sofia

Riess+2022

• Geometric indicators to calibrate the Period-Luminosity (PL) relations of Classical Cepheids

The first step to H₀

Period Luminosity relation

Teresa Sicignano

5

25/05/2026 CosmoVerse@Sofia

Period Luminosity relation

• From Cambridge – with humanistic background;

​

• Worked as a “Harvard computer” in charge of cataloguing the stars from the photographic plates;

​

• Measured the brightness of the variable stars.

​

Teresa Sicignano

6

25/05/2026 CosmoVerse@Sofia

Period Luminosity relation

Leavitt worked as a “Harvard computer” in charge of cataloguing the stars from the photographic plates.

Courtesy of Louise Breuval

Credit: Andrew Lockwood

Teresa Sicignano

7

25/05/2026 CosmoVerse@Sofia

Period Luminosity relation

Leavitt 1912

Courtesy of Louise Breuval

Teresa Sicignano

8

25/05/2026 CosmoVerse@Sofia

Period Luminosity relation

Breuval + 2025

Leavitt

OGLE

Leavitt 1912

Teresa Sicignano

9

25/05/2026 CosmoVerse@Sofia

Period Luminosity relation

Leavitt 1912

Teresa Sicignano

10

25/05/2026 CosmoVerse@Sofia

The Leavitt Law Over the XX^th century

• 1908 : Period Luminosity relation discovered by Miss Leavitt.

​

• 1912 : Publication of the results by Leavitt & Pickering in the Harvard Circular.

​

• 1924 : Leavitt was proposed for the Nobel Prize.

​

• 1923-25 : Leavitt Law serves Hubble to measure distances of astronomical objects, inside and outside our Galaxy.

​

• 1929 : Hubble law, the universe is expanding.

​

• 1998 : The Universe is accelerating!

​

​

Teresa Sicignano

11

25/05/2026 CosmoVerse@Sofia

Period Luminosity in the Great Debate

Hubble 1925

Teresa Sicignano

12

25/05/2026 CosmoVerse@Sofia

Period Luminosity in the Great Debate

Hubble, Washington, 1927

Teresa Sicignano

13

25/05/2026 CosmoVerse@Sofia

Only Period Luminosity?

Hubble 1925

Sandage 1958

Log P

Log Lum

The Period-Luminosity-Color relation

Teresa Sicignano

14

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

15

25/05/2026 CosmoVerse@Sofia

Teresa Sicignano

16

25/05/2026 CosmoVerse@Sofia

Pulsating stars are intrinsic variables showing periodic variations.

In the simplest case they are radial pulsators.

Pulsating variable stars

The basic driver of pulsation are changes to opacity:

​

Teresa Sicignano

17

25/05/2026 CosmoVerse@Sofia

Pulsating variable stars

The Period-Luminosity-Color relation

Teresa Sicignano

18

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

19

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

20

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

21

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

22

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

23

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

24

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

25

25/05/2026 CosmoVerse@Sofia

Consider the star as a self-gravitating sphere undergoing pulsations

​

The Period-Luminosity-Color relation

Teresa Sicignano

26

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

27

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

28

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

29

25/05/2026 CosmoVerse@Sofia

The Period-Luminosity-Color relation

Teresa Sicignano

30

25/05/2026 CosmoVerse@Sofia

P is a function of M, L, and T_eff (and chemical composition)

​

M is not independent

The Period-Luminosity-Color relation

Teresa Sicignano

31

25/05/2026 CosmoVerse@Sofia

P is a function of M, L, and T_eff (and chemical composition)

​

The Period-Luminosity-Color relation

Teresa Sicignano

32

25/05/2026 CosmoVerse@Sofia

P (M, L, and T_eff -and chemical composition)

​

Madore & Freedman1991

The Period-Luminosity-Color relation

Teresa Sicignano

33

25/05/2026 CosmoVerse@Sofia

Madore & Freedman1991

With observable quantities:

The Period-Luminosity-Color relation

Teresa Sicignano

34

25/05/2026 CosmoVerse@Sofia

Madore & Freedman1991

With observable quantities:

The PLC relation and its projections

Teresa Sicignano

35

25/05/2026 CosmoVerse@Sofia

Madore & Freedman1991

With observable quantities:

• The PLC relation holds for each individual Cepheid: period + color → absolute magnitude → distance

​

​

• The PL relation is obtained by averaging over the color extension of the instability strip

The PLC relation and its projections

Teresa Sicignano

36

25/05/2026 CosmoVerse@Sofia

Madore & Freedman1991

With observable quantities:

• The PLC relation holds for each individual Cepheid: period + color → absolute magnitude → distance

​

​

• The PL relation is obtained by averaging over the color extension of the instability strip

Standard(izable) Candle

The Instability Strip

Teresa Sicignano

37

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Teresa Sicignano

38

25/05/2026 CosmoVerse@Sofia

The physics behind the scenes…

The Pulsation mechanism

Teresa Sicignano

39

25/05/2026 CosmoVerse@Sofia

Only when the ionization regions are deep enough, the mass involved allows pulsation driving mechanisms to prevail over dumping.

�

When the quenching effect due to convection prevails, pulsation is no longer efficient.

�

The Pulsation mechanism

Teresa Sicignano

40

25/05/2026 CosmoVerse@Sofia

Only when the ionization regions are deep enough, the mass involved allows pulsation driving mechanisms to prevail over dumping.

�

When the quenching effect due to convection prevails, pulsation is no longer efficient.

�

BLUE EDGE OF THE INSTABILITY STRIP

RED EDGE OF THE INSTABILITY STRIP

Path to the Instability Strip

​

​

​

​

Teresa Sicignano

41

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Path to the Instability Strip

​

​

​

​

Teresa Sicignano

42

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Path to the Instability Strip

​

​

​

​

Teresa Sicignano

43

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Path to the Instability Strip

​

​

​

​

Teresa Sicignano

44

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Type II Cepheids:

BLHer & WVir Pop II; low mass;

1-4 & 4-20 days.

RVTau t > 2 Gyr; intermediate mass; 20-100 days.

Path to the Instability Strip

​

​

​

​

Teresa Sicignano

45

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Type II Cepheids:

BLHer & WVir Pop II; low mass;

1-4 & 4-20 days.

RVTau t > 2 Gyr; intermediate mass; 20-100 days.

Path to the Instability Strip

​

​

​

​

Teresa Sicignano

46

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Type II Cepheids:

BLHer & WVir Pop II; low mass;

1-4 & 4-20 days.

RVTau t > 2 Gyr; intermediate mass; 20-100 days.

Period Luminosity relations of pulsating stars in the LMC

Teresa Sicignano

47

25/05/2026 CosmoVerse@Sofia

Data from Cusano+2021, Ripepi+2022, Sicignano+2024,+2025

• Astronomers have used PL, but… if we want 1% precision on the Hubble constant, we must investigate every source of dispersion and systematics.

​

Teresa Sicignano

48

25/05/2026 CosmoVerse@Sofia

Limitations of the PL relation

• Astronomers have used PL, but… if we want 1% precision on the Hubble constant, we must investigate every source of dispersion and systematics.

​

​

• Width of the PL
• Extinction
• Metallicity

Teresa Sicignano

49

25/05/2026 CosmoVerse@Sofia

Limitations of the PL relation

• PL has a finite width → average goes with ~ square root of N.

​

• PL is a projection of the Period-Luminosity-Color.

Teresa Sicignano

50

25/05/2026 CosmoVerse@Sofia

• Width of the PL

• Extinction

• Stellar light is partially absorbed and scattered by dust and gas in the interstellar medium.

​

• This effect is called interstellar extinction.

​

• Extinction reduces the observed brightness of astronomical objects.

​

• It also alters the observed colors of stars.

​

​

Teresa Sicignano

51

25/05/2026 CosmoVerse@Sofia

• Extinction

• Interstellar gas: absorb & radiate​
• Interstellar dust: scatter​ (& radiate)​

​

Teresa Sicignano

52

25/05/2026 CosmoVerse@Sofia

• Extinction

Teresa Sicignano

53

25/05/2026 CosmoVerse@Sofia

• Extinction

Teresa Sicignano

54

25/05/2026 CosmoVerse@Sofia

• Extinction is stronger at shorter wavelengths ( blue light).
• Longer wavelengths (red/infrared) are less affected.

​

• As a result, stars appear redder and fainter than they actually are.

• Extinction

Teresa Sicignano

55

25/05/2026 CosmoVerse@Sofia

• Extinction is stronger at shorter wavelengths ( blue light).
• Longer wavelengths (red/infrared) are less affected.

​

• As a result, stars appear redder than they actually are.

Skowron 2022

From the PLC to the Period Wesenheit

Teresa Sicignano

56

25/05/2026 CosmoVerse@Sofia

Riess+2018

• Metallicity

Teresa Sicignano

57

25/05/2026 CosmoVerse@Sofia

Adapted from De Somma+2024

Adapted from Trentin+2024

P is a function of M, L, and T_eff (and chemical composition)

Theory

Observations

• Metallicity

Teresa Sicignano

58

25/05/2026 CosmoVerse@Sofia

P is a function of M, L, and T_eff (and chemical composition)

Breuval + 2024

STAY TUNED FOR THE EVENING LESSON!!

Not only Classical Cepheids…

Teresa Sicignano

59

25/05/2026 CosmoVerse@Sofia

Other pulsating stars that populate the instability strip can be used to establish an alternative first step of the extragalactic distance scale.

Data from Cusano+2021, Ripepi+2022, Sicignano+2024,+2025

WHERE WE ARE:

Teresa Sicignano

60

25/05/2026 CosmoVerse@Sofia

​

• Cepheids as distance indicators

​

• Pulsation mechanism are well known

​

• PL is a projection of PLC - PW is a PLC, with a special coefficient

​

• Extinction and metallicity matter

​

• Precision cosmology is systematics-limited

TYPE II AND ANOMALOUS CEPHEIDS

Teresa Sicignano

61

25/05/2026 CosmoVerse@Sofia

Type II Cepheids:

BLHer & WVir Pop II; low mass;

1-4 & 4-20 days.

RVTau t > 2 Gyr; intermediate mass; 20-100 days.

Bhardwaj+2022

HOW TO BUILD A PL/PW RELATION

Teresa Sicignano

62

25/05/2026 CosmoVerse@Sofia

HOW TO BUILD A PL/PW RELATION

• Recognize variable stars

Teresa Sicignano

63

25/05/2026 CosmoVerse@Sofia

• Recognize variable stars

Teresa Sicignano

64

25/05/2026 CosmoVerse@Sofia

OGLE

The Optical Gavitational Lensing Experiment

GAIA

High accuracy astrometric survey mission

• Recognize variable stars

Teresa Sicignano

65

25/05/2026 CosmoVerse@Sofia

OGLE

The Optical Gavitational Lensing Experiment

GAIA

High accuracy astrometric survey mission

RUBIN OBSERVATORY

Transient and Variable Sky Survey

HOW TO BUILD A PL/PW RELATION

• Recognize variable stars

​

Teresa Sicignano

66

25/05/2026 CosmoVerse@Sofia

HOW TO BUILD A PL/PW RELATION

• Recognize variable stars

​

• Measure the periods

Teresa Sicignano

67

25/05/2026 CosmoVerse@Sofia

• Measure the periods

Teresa Sicignano

68

25/05/2026 CosmoVerse@Sofia

The period is the time difference between two minima ( or maxima) of the magnitude of a star.

​

The more the light curve of the star is well sampled, the better we know the period.

​

The PL relation is a powerful tool because we the period is measured with an uncertainty much smaller than all other quantities involved.

​

​

• Measure the periods

Teresa Sicignano

69

25/05/2026 CosmoVerse@Sofia

• Measure the periods

Teresa Sicignano

70

25/05/2026 CosmoVerse@Sofia

HOW TO BUILD A PL/PW RELATION

• Recognize variable stars

​

• Measure the periods

​

Teresa Sicignano

71

25/05/2026 CosmoVerse@Sofia

HOW TO BUILD A PL/PW RELATION

• Recognize variable stars

​

• Measure the periods

​

• Measure the mean-magnitudes

Teresa Sicignano

72

25/05/2026 CosmoVerse@Sofia

• Measure the mean-magnitude

Teresa Sicignano

73

25/05/2026 CosmoVerse@Sofia

• Measure the mean-magnitudes

Teresa Sicignano

74

25/05/2026 CosmoVerse@Sofia

Flux arrived at the instrument

Zero-point flux according to the chosen photometric system

Teresa Sicignano

75

25/05/2026 CosmoVerse@Sofia

Flux arrived at the instrument

Zero-point flux according to the chosen photometric system

Bessell 2005

• Measure the mean-magnitudes

Teresa Sicignano

76

25/05/2026 CosmoVerse@Sofia

Sicignano + 2024

HOW TO BUILD A PL/PW RELATION

• Recognize variable stars

​

• Measure the periods

​

• Measure the mean-magnitudes

Teresa Sicignano

77

25/05/2026 CosmoVerse@Sofia

HOW TO BUILD A PL/PW RELATION

Teresa Sicignano

78

25/05/2026 CosmoVerse@Sofia

HOW TO BUILD A PL/PW RELATION

Teresa Sicignano

79

25/05/2026 CosmoVerse@Sofia

Sicignano + 2024

HOW TO BUILD A PL/PW RELATION

Teresa Sicignano

80

25/05/2026 CosmoVerse@Sofia

Sicignano + 2024

CALIBRATION WITH GAIA PARALLAXES

Teresa Sicignano

81

25/05/2026 CosmoVerse@Sofia

Teresa Sicignano

82

25/05/2026 CosmoVerse@Sofia

CALIBRATION WITH GAIA PARALLAXES

Teresa Sicignano

83

25/05/2026 CosmoVerse@Sofia

CALIBRATION WITH GAIA PARALLAXES

CALIBRATION WITH GAIA PARALLAXES

Teresa Sicignano

84

25/05/2026 CosmoVerse@Sofia

SANITY CHECK: THE LMC DISTANCE

Teresa Sicignano

85

25/05/2026 CosmoVerse@Sofia

Sicignano + 2025

APPLICATION: THE TIP OF THE RED GIANT BRANCH

Teresa Sicignano

86

25/05/2026 CosmoVerse@Sofia

Lengen + 2026

Cruz-Reyes + 2026

Teresa Sicignano

87

25/05/2026 CosmoVerse@Sofia

TAKE HOME MESSAGES

• The discovery and the role of the Leawitt Law

​

Teresa Sicignano

88

25/05/2026 CosmoVerse@Sofia

TAKE HOME MESSAGES

• The discovery and the role of the Leawitt Law

​

• The Period-Luminosity- Color and its limitation

​

Teresa Sicignano

89

25/05/2026 CosmoVerse@Sofia

TAKE HOME MESSAGES

• The Period-Luminosity- Color and its limitation

​

Teresa Sicignano

90

25/05/2026 CosmoVerse@Sofia

• Metallicity?

TAKE HOME MESSAGES

• The discovery and the role of the Leawitt Law

​

• The Period-Luminosity- Color and its limitation

​

• The physics of a pulsating star

​

Teresa Sicignano

91

25/05/2026 CosmoVerse@Sofia

TAKE HOME MESSAGES

• The physics of a pulsating star

​

Teresa Sicignano

92

25/05/2026 CosmoVerse@Sofia

TAKE HOME MESSAGES

• The discovery and the role of the Leawitt Law

​

• The Period-Luminosity- Color and its limitation

​

• The physics of a pulsating star

​

• How to measure distances with pulsating star

Teresa Sicignano

93

25/05/2026 CosmoVerse@Sofia

Teresa Sicignano

94

25/05/2026 CosmoVerse@Sofia

THE FUTURE IS PULSATING

Teresa Sicignano

95

25/05/2026 CosmoVerse@Sofia

teresa.sicignano@inaf.it

Thank you for your attention!

BACKUP SLIDES

Teresa Sicignano

97

25/05/2026 CosmoVerse@Sofia

An alternative self-consistent stellar route to the Hubble constant

Teresa Sicignano

98

25/05/2026 CosmoVerse@Sofia

Exploitation of the VMC-Deep and TNG-REM photometry for Type II and Anomalous Cepheids.

Quantify the effect of metallicity on the zero point and slope of PL relation through high-resolution spectroscopic abundances from the 4MOST 1001MC survey and UVES spectra.

Compare the distance scales produced for different kinds of pulsating stars and look into the reasons for any discrepancies.

Calibration through Gaia DR4 parallaxes.

Teresa Sicignano

99

25/05/2026 CosmoVerse@Sofia

TYPE II AND ANOMALOUS CEPHEIDS IN THE MAGELLANIC CLOUDS

• VMC is an ESO public survey (P.I. M.-R. Cioni)
• Observations in YJKs with VIRCAM@VISTA 4 m (Paranal, Chile)
• Data reduction with the VISTA Data Flow System (VDFS) pipeline at CASU (Cambridge Astronomical Survey Unit)
• Catalogues handling through the Vista Science Archive (VSA)

​

339 T2Cs and 198 ACs with VMC photometry in the Magellanic Clouds

TEMPLATE FITTING TO THE DATA

Teresa Sicignano

100

25/05/2026 CosmoVerse@Sofia

To construct the PL/PW it is necessary to calculate the intensity-averaged magnitudes, using the tecnique of templetes.

​

TYPE II CEPHEIDS

Teresa Sicignano

101

25/05/2026 CosmoVerse@Sofia

OBSERVED PERIOD – LUMINOSITY RELATIONS

Sicignano+2024,

A&A, 685, A41.

LIGHT CURVES

TYPE II CEPHEIDS

Teresa Sicignano

102

25/05/2026 CosmoVerse@Sofia

FITTED PERIOD – LUMINOSITY AND PERIOD - WESENHEIT RELATIONS

Sicignano+2024,

A&A, 685, A41.

IMPACT ON THE DISTANCE SCALE

Teresa Sicignano

103

25/05/2026 CosmoVerse@Sofia

The distance moduli of GGCs appear overestimated up to 3%.

Comparison between our distance moduli and those by Baumbgardt & Vasiliev 21

Sicignano+2024,

A&A, 685, A41.

ANOMALOUS CEPHEIDS

Teresa Sicignano

104

25/05/2026 CosmoVerse@Sofia

OBSERVED PERIOD – LUMINOSITY RELATIONS

Sicignano

+2025, A&A,

under revision.

LIGHT CURVES

Teresa Sicignano

105

25/05/2026 CosmoVerse@Sofia

Riess+2022

• Geometric indicators to calibrate the Period-Luminosity (PL) relations of Cepheids

Freedman+ 2021

IMPACT ON THE DISTANCE SCALE

ANOMALOUS CEPHEIDS

Teresa Sicignano

106

25/05/2026 CosmoVerse@Sofia

FITTED PERIOD – LUMINOSITY AND PERIOD WESENHEIT RELATIONS

Sicignano

+2025, A&A,

under revision.

FIRST TIME: FUNDAMENTALIZATION OF FIRST OVERTONE ACs

ANOMALOUS CEPHEIDS

Teresa Sicignano

107

25/05/2026 CosmoVerse@Sofia

FITTED PERIOD – LUMINOSITY AND PERIOD-WESENHEIT RELATIONS

Sicignano

+2025, A&A,

under revision.

LMC/SMC RELATIVE DISTANCE

Teresa Sicignano

108

25/05/2026 CosmoVerse@Sofia

Sicignano+2025, under review.

DRACO

Teresa Sicignano

109

25/05/2026 CosmoVerse@Sofia

Sicignano

+2025,A&A,

under review.

Future prospectives

Teresa Sicignano

110

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

{

• TYPE II CEPHEIDS DIRECTLY IN SN Ia HOST

LSST VERA RUBIN AND NANCY GRACE ROMAN SPACE TELESCOPES.

Sicignano+,

in prep.

A self-consistent stellar route to the Hubble constant

Teresa Sicignano

111

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

{

Exploitation of the VMC-Deep and TNG-REM photometry for Type II and Anomalous Cepheids.

Quantify the effect of metallicity on the zero point and slope of PL relation through high-resolution spectroscopic abundances from the 4MOST 1001MC survey and UVES spectra.

Compare the distance scales produced for different kinds of pulsating stars and look into the reasons for any discrepancies.

Calibration through Gaia DR4 parallaxes.

Teresa Sicignano

112

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

{

Exploitation of the VMC-Deep and TNG-REM photometry for Type II and Anomalous Cepheids.

Quantify the effect of metallicity on the zero point and slope of PL relation through high-resolution spectroscopic abundances from the 4MOST 1001MC survey and UVES spectra.

Compare observational data with newly developed stellar pulsation models.

SESAME: Synergies between ESO�Spectroscopic And photoMEtric surveys

Path to the Instability Strip

Stars cross the IS at different evolutionary phases:

​

• Classical Cepheids 3 crossings during central Helium burning phase.

​

• Type II Cepheids: BLHer post HB, WVir during AGB, RVTau post AGB.

​

• Anomalous Cepheids during core Helium burning phase.

​

​

​

Teresa Sicignano

113

25/05/2026 CosmoVerse@Sofia

Bhardwaj+2022

Teresa Sicignano

114

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

{

• ZERO POINT CALIBRATION

TYPE II CEPHEIDS:

THEORY? OBSERVATIONS?

Di Criscienzo+2007

• Metallicity spanning of Pop II stars in the LMC.

​

• Predicted PL dependence on metallicity.

​

• 7 Galactic T2C show a dependence

& No dependence in GGCs.

​

​

A self-consistent stellar route to the Hubble constant

Teresa Sicignano

115

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

{

• ZERO POINT CALIBRATION

RRLYRAE THEORY AND OBSERVATIONS

Adapted from

Marconi+2015

Adapted from

Bhardwaj+2023

Teresa Sicignano

116

25/05/2026 CosmoVerse@Sofia

Teresa Sicignano

117

25/05/2026 CosmoVerse@Sofia

The extragalactic distance scale. The three steps to H₀

Teresa Sicignano

118

25/05/2026 CosmoVerse@Sofia

A self-consistent stellar route to the Hubble constant

Teresa Sicignano

119

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

MASTER

THESIS

{

PHD

PROJECT

• ZERO POINT CALIBRATION

TYPE II CEPHEIDS:

THEORY? OBSERVATIONS?

Gratton+2004

• Metallicity spanning of RRLyrae (Pop II) in LMC.

A self-consistent stellar route to the Hubble constant

Teresa Sicignano

120

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

MASTER

THESIS

{

PHD

PROJECT

• ZERO POINT CALIBRATION

TYPE II CEPHEIDS:

THEORY? OBSERVATIONS?

Wielgorski+2022

• Metallicity spanning of RRLyrae (Pop II) in LMC.

​

• Predicted PL dependence on metallicity for BLHer.

​

• 7 Galactic T2C show a dependence

& No dependence in GGCs.

​

WHY TYPE II CEPHEIDS?

Teresa Sicignano

121

25/05/2026 CosmoVerse@Sofia

Adapted from Beaton+2016

Teresa Sicignano

122

25/05/2026 CosmoVerse@Sofia

Riess+2022

The first step to H₀

• Geometric indicators to calibrate the Period-Luminosity (PL) relations of Cepheids

Riess+2024

TYPE II CEPHEIDS

Teresa Sicignano

123

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

TYPE II CEPHEIDS

Teresa Sicignano

124

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

MASTER

THESIS

Evolution of Population II stars

Teresa Sicignano

125

25/05/2026 CosmoVerse@Sofia

BASIC ASTROPHYSICAL CONCEPTS

Pulsating variable stars

Teresa Sicignano

126

25/05/2026 CosmoVerse@Sofia

Type II Cepheids

Teresa Sicignano

127

25/05/2026 CosmoVerse@Sofia

Anomalous Cepheids

Teresa Sicignano

128

25/05/2026 CosmoVerse@Sofia

Pulsation mechanism

Trapped energy becomes pulsational work.

• γ Mechanism: an initial (stochastic) contraction of the star leads to an increase in density. In the ionization regions, at the same time, there is a temperature increase, but less than the same scenario without ionization. Since the luminosity L goes as T 4 , its variation will also be smaller: there is an energy entrapment. The excess energy, during the next expansion phase, goes into pulsational work.
• Κ Mechanism: In a stellar interior, normally the opacity decreases during a contraction, according to Kramer’s law (κ ∼ ρT −3.5 ) producing a loss of heat. In the ionization regions, due to the interaction of radiation with matter, small increases in temperature cause a large increase in opacity (the temperature exponent in Kramer’s law becomes positive). This again means trapping of energy, which will be converted into pulsational work.

​

Teresa Sicignano

129

25/05/2026 CosmoVerse@Sofia

PULSATING STARS IN THE MAGELLANIC CLOUDS

Teresa Sicignano

130

25/05/2026 CosmoVerse@Sofia

Teresa Sicignano

131

25/05/2026 CosmoVerse@Sofia

Teresa Sicignano

132

25/05/2026 CosmoVerse@Sofia

Type II and Anomalous Cepheids

Teresa Sicignano

133

25/05/2026 CosmoVerse@Sofia

PULSATING STARS IN THE MAGELLANIC CLOUDS

Period-luminosity-colour and �period-luminosity

Teresa Sicignano

134

25/05/2026 CosmoVerse@Sofia

PULSATING STARS IN THE MAGELLANIC CLOUDS

Template derivation

But can all observed light curves be fitted?

Teresa Sicignano

135

25/05/2026 CosmoVerse@Sofia

FROM THE VISTA TELESCOPE TO THE MULTIBAND PHOTOMETRY

2. Calculate the average magnitude in each band:
• Fitting the light curve with some analytical functions.
• Transforming it into intensities.
• Calculate the average in intensity.
• Transforming the average intensity in average magnitude.

TYPE II CEPHEIDS

Teresa Sicignano

136

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

MASTER

THESIS

IMPACT ON THE DISTANCE SCALE

Teresa Sicignano

137

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

MASTER

THESIS

Comparison between our distances and those by Bhardwaj 2023

The distance moduli of GGCs appear overestimated up to 2%.

Sicignano+2024,

A&A, 685, A41.

IMPACT ON THE DISTANCE SCALE

Teresa Sicignano

138

25/05/2026 CosmoVerse@Sofia

• PERIOD –LUMINOSITY RELATION

​

• PRESENCE IN ANCHORS

​

• ZERO POINT CALIBRATION

​

• SN Ia HOST

​

{

MASTER

THESIS

― Slope LMC

• Galactic T2C

Sicignano+2024,

A&A, 685, A41.