Simulia Living Heart Project

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The Araucaria Project: Improving the cosmic distance scale Editorial board: Paulina Karczmarek, Megan Lewis, Grzegorz Pietrzyński Photo: All the authors, with the addition of Celeste Burgos and Michael Ramolla, are credited for the photos and images in this book.

Back Cover: Grzegorz Pietrzyński

Front cover: Merrillie Redden Photography – merrillie.com Layout, graphic design and printing: Wydawnictwo Aleksander Copyright© Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, Warsaw 2021

Copyright© Wydawnictwo Aleksander, Pułtusk 2021

Published by: Nicolaus Copernicus Astronomical Center

Wolfgang Gieren And Grzegorz Pietrzyński

A brief history of the Araucaria Project ...................................................................... 8 Paulina Karczmarek, Wolfgang Gieren, Grzegorz Pietrzyński Optical survey of classical Cepheids ......................................................................... 18

Wolfgang Gieren

Near-infrared follow-up of extragalactic classical Cepheids previously discovered in optical photometric surveys ................................................... 21

Marek Górski

Tip of the Red Giant Branch stars as distance indicators ............................................... 25

Marek Górski

Red Clump stars as distance indicators ..................................................................... 28

Bartłomiej Zgirski

Distance determinations based on carbon stars .......................................................... 31

Paulina Karczmarek

RR Lyrae stars as near-infrared distance indicators to Local Group galaxies ..................... 34

Piotr Wielgórski

Type II Cepheids as distance indicators .................................................................... 37

Louise Breuval

Impact of metallicity on the Leavitt Law .................................................................... 40

Ksenia Suchomska

Physical and orbital properties of Bulge late-type eclipsing binaries ................................ 43

Alexandre Gallenne

The role of interferometry in measuring high-precision distance and mass of binary stars ... 46

Grzegorz Pietrzyński

Distance to the Magellanic Clouds accurate to 1% from eclipsing binaries .......................... 49

Pierre Kervella

Multiplicity among classical Cepheids and RR Lyrae stars ............................................. 52

Gergely Hajdu

RR Lyrae stars in binary systems ............................................................................. 55

Bogumił Pilecki

High-precision astrophysics of Cepheids in binary systems .......................................... 58

Jesper Storm

The Baade-Wesselink technique and the effect of metallicity on the Leavitt law ................... 61

Nicolas Nardetto

The Nice contribution: The projection factor of Cepheids ............................................. 64

Rolf Kudritzki

Flux weighted gravity-luminosity relationship: A new distance indicator ........................ 67

Nicolas Nardetto

The Nice contribution: CHARA/VEGA and the SBCR of early-type stars ............................ 71

Mónica Taormina

Toward Early-type Eclipsing Binaries as Extragalactic Milestones .................................. 74

Fabio Bresolin

Metallicities of nearby galaxies ................................................................................ 77

Radosław Smolec

Modelling of classical pulsators ............................................................................... 80

Paulina Karczmarek And Radosław Smolec

Binary Evolution Pulsators – a new class of pulsating stars ............................................ 83

Weronika Narloch

Non-pulsating stars in the instability strip of classical Cepheids .................................... 86

Cezary Gałan

Atmospheric parameters and abundances of the eclipsing binary systems components from high-resolution spectra .................................................................................. 89

Weronika Narloch

Age-metallicity relation of star clusters in the Magellanic Clouds .................................... 92

Megan Lewis

Maser-derived parallax measurements of Miras ......................................................... 95

Nicolas Nardetto

The Nice contribution: The CHARA/SPICA interferometer ............................................ 97

Rolf Chini

Observatorio Cerro Armazones – the history, the present, and the future ....................... 100 Contributors ................................................................................................. 106 Beyond the Araucaria Project: hikes, social gatherings and conferences .............. 108

Up The Distance Scale. The Main Systematic

uncertainty in the HST Key Project seemed to be the largely unknown effect of environmental properties, mostly metallicity, on the luminosities of classical Ce­ pheid variables which had been extensively used by the Key Project to determine the distances to supernova Ia host galaxies, and to calibrate these SN Ia as standard candles reaching out into the unperturbed Hubble flow where the Hubble constant can be reliably determined.

Our conclusion was that it was absolutely necessary to com­ plement the HST Key Project work with a thorough determi­ nation of environmental effects on Cepheids, and a more ac­ curate distance to the Large Magellanic Cloud which served as the anchor galaxy to the HST Key Project SN Ia host gal­ axy distances. The perfect laboratories to do this were the galaxies in the nearby Local and Sculptor Groups which offer a wide range of types, and environmental properties. Eventually, we thought that it would be of great interest to not only carry out studies aiming at improving classical Cepheids as standard candles, but also other stellar distance indicators like RR Lyrae stars,

Of The Araucaria Project

1 Universidad de Concepción, Departamento de Astronomia, Casilla 160-C, Concepción, Chile 2 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland In early 2000, Grzegorz Pietrzyński started to work with Wolfgang Gieren in Concepción. During this time the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale by Wendy Freedman and collaborators was about to finish, and the topic of distance determina­ tion was again, and very strongly, in the focus of the international astronomical community.

red clump stars, the Tip of the Red Giant Branch, Type II Cepheids, eclipsing binaries, and others. Another key consideration was to carry the stellar methods of distance determination, traditionally calibrated in the optical, to the near-infrared regime where extinction was much less a problem than in the optical spectral region, and where some previous theoretical studies had suggested that environmental dependences might be smaller as well. It was good timing for such studies Fig. 1. Araucaria tree in Conguillío National Park.

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because excellent near-infrared cameras had just be­ come available at several observatories around the be­ ginning of the 21st century. When it became clear to us that we were starting a very ambitious and long-lasting project, we needed a name for it, and decided to call it the “Araucaria Project”, making reference to the place the first ideas were born (Araucaria trees are the most beautiful native trees in southern Chile).

In The Same Year, 2000, Wolfgang And Grzegorz

visited Rolf Kudritzki in Munich. Rolf was a leading expert on blue supergiant stars and had some ideas

In Mind About How To Use These Extremely Luminous

objects, visible in very distant galaxies, for distance

Determination. Wolfgang And Grzegorz Were Working

at this time, together with Pascal Fouqué, who was a visitor at the European Southern Observatory’s (ESO) branch in Chile, on a survey for Cepheids in the beau­ tiful spiral galaxy NGC 300 in the Sculptor Group us­

Ing Images Obtained With The Eso/Mpi Wide-Field

Imager at the 2.2-m telescope on La Silla, and it was evident from the blue images that NGC 300 contained a lot of bright blue stars. So we quickly agreed with Rolf that it would be a very nice project to test his spectro­ scopic method (which eventually became known as the “Flux-Weighted Gravity-Luminosity Relationship”) to determine distances using blue supergiants, and check their correctness by comparison with the distance we were about to determine from the NGC 300 Cepheids.

This way the Araucaria Project group saw its first ex­ pansion, by integrating Rolf Kudritzki and his close collaborators, Fabio Bresolin and Miguel Urbaneja in the project, who were to strongly contribute to the sci­ entific success of the project over the next two decades.

Lulu, 2006. From Left: Fabio,

Wolfgang, Miguel, Rolf.

At The Same Time, Giuseppe Bono From Rome Ob­

servatory joined the project which opened up the op­ portunity to complement the observational work with theoretical model results and predictions, with mutual benefits for both approaches to study and understand pulsating variable stars. Another member of the Ar­ aucaria Project who joined our group in its very first years was Jesper Storm from AIP Potsdam. He, Wolf­

Gang And Pascal Fouqué Had Been Working For Some

years together to develop a near-infrared version of the classical Baade-Wesselink method, the Infrared Surface

Brightness (Irsb) Technique, And Now Went On With

this work as part of the Araucaria Project. Another two key members of the Project joined us some years later:

Ian Thompson From The Observatories Of The Carne­

gie Institute of Washington, and Darek Graczyk, who joined our team in Concepcion. Ian and Darek became interested in collaborating with the Araucaria Project group when we started to extensively observe special

Eclipsing Binary Systems In The Magellanic Clouds

which were composed of pairs of red giants. These very rare systems had been discovered by the OGLE Project

At Las Campanas Observatory And Needed Radial Ve­

locity follow-up observations to confirm their physical reality, and derive their properties and distances. Ian not only added his experience in eclipsing binary stud­ ies to the Project but also, through his special access to the Magellan telescopes at Las Campanas Observatory in Chile, helped to start very ambitious observing cam­ paigns on these faint objects which, after a decade or more of continued data collection, led to the most ac­ curate-ever distance determination to both Magellanic Clouds. Darek was taking care of precision modeling of our binary systems.

Fig. 3. Araucaria Meeting in Potsdam, Germany, 2010.

Through The Years, A Number Of Very Talented And

motivated young scientists joined the Araucaria Proj­ ect. Igor Soszynski from Warsaw worked for two years

In Concepción With Wolfgang And Grzegorz (2004-

2006) on classical Cepheids; other postdoctoral fellows

Working In Concepción Were Olaf Szewczyk, Nicolas

Nardetto and Marek Górski. Olaf started to work on the improvement of RR Lyrae stars as distance indi­ cators using near-infrared photometry, and eventually obtained good distance determinations to both Mag­ ellanic Clouds. Nicolas was working on the modeling of classical Cepheid atmospheres mainly to better un­ derstand how observed Cepheid radial velocities could be transformed to the pulsation velocities of these vari­ ables, a central problem in the IRSB method to be over­ come in order to yield reliable distances from this tech­ nique. Marek was working on red clump stars and the

Trgb Method To Improve Our Understanding Of How

these stellar methods of distance determination de­ pend on age and metallicity, and also became involved in the IRSB work led by Wolfgang and Jesper.

In 2006 Grzegorz Decided To Go Back To Poland. It

was a very difficult decision because he was extremely happy working and living in beautiful southern Chile.

However This Decision Turned Out To Have A Strong

positive impact on the Araucaria Project. Thanks to generous support from the Foundation of Polish Sci­

Warsaw. Many Talented Researchers Joined Our Team

thanks to these funds including Bogumił Pilecki, Pauli­ na Karczmarek, Piotr Konorski, Mónica Taormina and Ksenia Suchomska. Since then the Araucaria team has been able to expand research on several different kinds of eclipsing binaries and even pulsating stars in eclips­ ing binaries.

Fig. 4. Araucaria Meeting in Nice, France, 2013.

Darek Graczyk Has Been Leading The Project On

eclipsing binaries in the Solar neighborhood, and also kept working on binaries in the Magellanic Clouds. Bogumił managed to very carefully model several clas­ sical and Type II Cepheids in eclipsing binaries lead­ ing to the determination of their physical parameters

With Unprecedented Accuracy. Grzegorz And Darek

discovered a very special pulsating star which mimics a classical RR Lyrae star but is the product of binary evolution. Mónica obtained very interesting results on

Very Promising Eclipsing-Binary Systems Composed

of early-type stars. Ksenia dedicated a lot of work to studying Milky Way eclipsing binaries composed of gi­ ants. Paulina performed simulations to study in detail the evolution of pulsating stars in binary systems. We also gained another important collaborator, an expert on eclipsing binaries - Pierre Maxted. All these scientif­ ic projects opened a new era in the Araucaria project.

Most of them have been continued until today (2021) and resulted in several dozen publications, including 4 papers published in the prestigious Nature journal.

Another Important Expansion Of Our Research Line

was possible due to the start of a close collaboration with Pierre Kervella and Nicolas Nardetto in 2003, and Alexandre Gallenne in 2008. Thanks to their expertise in interferometry we were now able to conduct several new projects which gave us the opportunity to improve Fig. 5. Araucaria Meeting in the Royal Restaurant Wierzynek in Kraków, Poland, 2017.

A Brief History Of The Araucaria Project

on previous results. By the year 2018, we managed to very precisely measure angular diameters of some 50 red clump stars which, in tandem with high quality near-IR data collected for the same stars at the South African Astronomical Observatory, that allowed us to improve the calibration of the surface brightness-col­ or relationship for stars in the Cepheid color range, an effort which complemented our hectic efforts on eclipsing binaries very well. This is just one very good example of the strong synergy we now had within our group. Alexandre and Pierre worked on several very interesting astrometric binary systems in the Milky Way containing classical Cepheids, work which com­ plements our efforts to study Cepheids in eclipsing bi­ naries in the Large Magellanic Cloud.

All These Advances Allowed Us To Formulate More

ambitious science goals and apply for, and eventual­ ly obtain, important new funding from the European Research Council (ERC) in the form of a prestigious ERC advanced grant. This grant, realized from 2016 to 2021, allowed us to build an even stronger science

Team. Many Very Valuable Collaborators Joined Our

project including Weronika Narloch, Piotr Wielgórski, Bartek Zgirski, Gergely Hajdu, Wojtek Pych, Mikołaj

Kałuszyński, Czarek Gałan, Gonzalo Rojas, Megan

Lewis, Louise Breuval, Simon Borgniet, Boris Trahin,

And Behnam Javanmardi. Louise, Piotr, And Bartek

worked mostly on pulsating stars. They produced very precise fiducial period-luminosity relations for classi­ cal Cepheids, Type II Cepheids, and RR Lyrae stars in different environments. Gergely studied mostly RR Ly­ rae stars, and Megan Miras in the Milky Way. Mikołaj and Gonzalo modeled eclipsing binaries. Simon, Boris, and Wojtek worked on spectroscopic data, while Beh­ nam used archival Hubble Space Telescope data to in­ dependently determine the Cepheid distance to one of the key SN host galaxies, using new algorithms which allowed a valuable check on the results and methods applied by previous research teams.

Meanwhile we realized that, in order to take full ad­ vantage of the huge amount of precision data from sat­ ellite missions like Gaia, TESS, or Kepler to improve the extragalactic distance scale, one needs to complement these satellite data with very precise ground-based data. The only way to do that given the extremely large amount of data needed, is to collect them with dedi­ cated telescopes at a very good site. A breakthrough in our project occurred around 2016 when we started to closely collaborate with Rolf Chini’s group who oper­ ated their own telescopes at Cerro Armazones in north­

Ern Chile. Our Team Members Not Only Had A Unique

opportunity to obtain the required observations but

From Bochum Including Martin Hass, Michael Ramo­

lla, Francisco Pozo, Moritz Heckstein, Christian West­

Hues, Catalina Sobrino Figuaredo, Sadegh Noorozi,

and others. This particularly successful collaboration allowed us to start a few instrumental projects funded by the Polish Ministry of Science and Higher Educa­ tion, including building a few new modern telescopes.

On the basis of all our previous work we successful­ ly applied for an ERC Synergy grant on the continua­ tion and extension of the Araucaria Project. Within this grant which started in late 2021, we will build a 2.5m telescope which will extend enormously our observing capabilities, and together with our smaller telescopes, will enable us to collect an enormous amount of high quality photometric and spectroscopic data for eclips­ ing binaries, pulsating stars, and also for Active Galac­ tic Nuclei which offer a completely different and inde­ pendent route to build the extragalactic distance scale and extend it out to enormous distances, observing and measuring galaxies in an era when the Universe was only half as old as today. Therefore, our scientific panorama for the next 6 years looks extremely promis­ ing, paving the way to a solid measurement of Hubble constant, reaching a 1% accuracy, and even tracing the possible variation of its value over cosmic time.

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The Araucaria Project has so far produced about 200 papers in the most prestigious astronomical journals, and it is far beyond the scope of this article to mention all the science results it has produced over the two de­ cades since its start in 2000. A few highlights however

Should Be Mentioned Here:

- in 2001, the first successful survey for classical Ce­ pheids in NGC 300 was finished, pushing the number of known Cepheids in this galaxy from 18 to 117, and demonstrating that our discovery strategy was suc­

Cessful (Pietrzyński Et Al. 2002, Aj, 123, 789)

- in the same year, we discovered and obtained spec­ tra for a large number of blue supergiants in NGC 300 which were later used to calibrate blue supergiants as distance indicators (Bresolin et al. 2002, ApJ, 567, 277) - in 2003, a first study on environmental effects on red clump stars in optical and near-infrared bands was published (Pietrzyński et al. 2003, AJ, 125, 2494) - in 2005, the most accurate-ever (at the time) distance to a galaxy was derived for NGC 300 by applying the multiband optical/near-infrared photometric method developed by the Araucaria Project to its Cepheid vari­

Ables (Gieren Et Al. 2005, Apj, 628, 695)

- also in 2005, an in-depth pulsation modeling study of classical Cepheids yielded a period-age relation for these stars which up to now has become the standard reference in the field (Bono et al. 2005, ApJ, 621, 966) - in 2006 we published the first paper reporting results from an improved TRGB application to red giants in NGC 300, with data obtained with the Advanced Cam­ era for Surveys onboard the Hubble Space Telescope

Fig. 6. Araucaria Meeting In Concepción,

Chile, 2019.

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- in 2008, we obtained an accurate distance to the Large Magellanic Cloud using RR Lyrae stars in the near-in­

Frared (Szewczyk Et Al. 2008, Aj, 136, 272)

- also in 2008, the Araucaria Project published a defin­ itive calibration of the FGLR spectroscopic method for blue supergiants which was later used to determine the distances to most of the Araucaria Project target galax­ ies, with results mostly in excellent agreement with the Cepheid distances (Kudritzki et al. 2008, ApJ, 681, 269) - in 2009, a first paper analyzing one of the rare eclipsing binary systems in the LMC consisting of a pair of red gi­ ants was published, demonstrating the extraordinary po­ tential these systems have for a super-accurate distance determination (Pietrzyński et al. 2009, ApJ, 697, 862) - also in 2009, a study was published which compared the metallicities and metallicity gradients in NGC 300 as derived from young stars to those derived from H II regions, showing that within very small uncertainties the two methods yield identical results. This is a very important result in the context of distance determina­ tion, which needs information about metallicities in the target galaxies whose distances are to be measured

(Bresolin Et Al. 2009, Apj, 700, 309)

- in the same year, 2009, we published an accurate Ce­ pheid-based distance determination to the so-far most distant galaxy in our project, NGC 247 in the Sculptor

Group (Gieren Et Al. 2009, Apj, 700, 1141)

- in 2010, our group finished the analysis of the first classical Cepheid discovered in an eclipsing binary sys­ tem in the LMC. The paper reported the first dynamical mass determination of a classical Cepheid ever, accu­ rate to one percent, and in this way resolved the famous mass discrepancy problem for classical Cepheids show­ ing that the masses predicted by pulsation theory were correct (Pietrzyński et al. 2010, Nature, 468, 542) - in 2011, we published a re-calibration of the IRSB Baade-Wesselink method using for the first time a large sample of Large Magellanic Cloud Cepheids which set additional constraints on the calibration not available from Galactic Cepheids alone. This new calibration represented a very significant improvement over the

(Storm Et Al. 2011, A&A, 534, A94 And A95)

-in 2012, we published precision near-IR photometry of some 200 nearby red clump stars. This opened the road to improve the surface brightness-color relation for late-type stars and was a necessary condition to mea­ sure a 1% distance to the LMC in the future. (Laney et

Al. 2012, Mnras, 419, 103)

- also in 2012, we announced the discovery of a new class of pulsating stars, called Binary Evolution Pulsa­ tors. (Pietrzyński et al. Nature, 484, 75). Pulsating mod­ els for such stars were then developed by Araucaria scientist Radek Smolec et al. (2013, MNRAS, 428, 3034) while Paulina Karczmarek studied in detail possible evolutionary channels for such stars.

- in 2013 we presented a 2% distance determination to the LMC based on eclipsing binaries (Pietrzyński et al. 2013, Nature, 495, 56) which was the most precise-ev­ er distance measured to the LMC at the time. The dis­ tance to the SMC based on the same technique could be measured by our team with a precision of 3% (Graczyk et al. 2014, ApJ, 780, 59). These were the first very solid results from our decade-long (at that time!) observing efforts on this project and received outstanding atten­ tion from the international astronomical community.

- in 2014 we presented the results of the analysis of a very exotic eclipsing binary system which revealed that it is composed of two classical Cepheids (Gieren et al. 2014, 786, 80). It was (and still is) the first-ever firm­ ly established binary system with two Cepheid stars orbiting each other.

- in 2016 Gallenne et al. (A&A, 586, 35), presented a very precise and accurate distance measurement to the binary system TZ For, based on a combination of its astrometric and spectroscopic orbits.

- also in 2016 another important study on chemical abundances in the high-metallicity spiral galaxy M83 was published by Bresolin et al. (ApJ, 830, 64), while Kudritzki et al. (ApJ, 829, 70) reported another distance

Determination To The Galaxy Ngc 55 Using The Flgr

method.

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- in 2017 Nardetto et al. published an extensive theoret­ ical study on the p-factor problem in the Baade-Wes­ selink technique. The important conclusions reached in this paper were based on high-resolution spectroscop­ ic data for the Cepheid prototype star Delta Cephei

(A&A, 597, 73).

- also in 2017 a new distance determination to the For­ nax dwarf galaxy from near-IR photometry of RR Ly­ rae stars was announced by Paulina Karczmarek et al.

(AJ, 154, 263). This work concluded our long-term ef­ fort on determining improved distances to nearby gal­ axies from this method.

- also in 2017 a new and improved calibration of the FLGR method based on the detailed study of blue su­ pergiants in the LMC, was published by Urbaneja et al.

(Aj 154, 102).

- in 2018, an extremely precise determination of the physical parameters for six classical Cepheids mem­

Bers In Eclipsing Binary Systems In The Lmc Was

presented (Pilecki et al. ApJ, 862, 43). This work has yielded the most accurate masses and radii known to date for this important class of pulsating stars. It also concluded our long term efforts on studying classical Cepheids in eclipsing binaries.

- also in 2018 the effect of metallicity on the brightness of classical Cepheids was precisely estimated from a Baade-Wesselink analysis of Cepheids in the Milky Way, LMC and SMC (Gieren et al. A&A, 620, A99). For the first time, a very sizable sample of metal-poor Ce­ pheids in the SMC could be included in the analysis thanks to data which were obtained over five years as part of an ESO Large Programme.

- also in 2018 a new multi-band calibration of the ab­

Solute Magnitude Of The Trgb Was Published By Our

group (Górski et al. AJ, 156, 278). This work demon­ strated that the TRGB absolute magnitude can be pre­ cisely and fully empirically calibrated with multiband photometric data.

- in 2019 several results regarding the binarity fraction of Cepheids (Kervella et al. A&A, 623, 110, Gallenne et al. 2019, A&A, 622, A164) and RR Lyrae stars (Kervella et al. A&A, 623, A116) were published which strongly expanded our previous efforts on this topic.

- also in 2019 we published our most precise LMC dis­ tance from eclipsing binaries (Pietrzyński et al. Na­ ture, 567, 200). We managed to achieve a 1% precision, which makes the LMC the best anchor for the extraga­ lactic distance scale at the present time. This paper has been enthusiastically received by the community and has become the standard reference in both cosmologi­ cal studies and studies on astrophysical objects in the Large Magellanic Cloud.

- Also In 2019 And Based On Strömgren Photometry,

Narloch et al. (MNRAS, 489, 3285) detected many candi­ dates for non-pulsating stars located in the main Cephe­ id instability strip. The physical nature of these objects remains unclear so far but is potentially very interesting.

- also in 2019 the first paper on eclipsing binaries con­

Sisting Of Early Type Components Was Published By

Taormina et al. (ApJ, 886, 111). Apart from interesting results on the physical parameters of massive stars in the LMC, these results point towards the possibility to use such systems to extend the calibration of the sur­ face brightness-color relation to very early type, hot stars, opening a window for using bright, hot stars in galaxies to determine their distances rather precisely.

- in 2020 we published extended reddening maps of the

Lmc And Smc Which Are Very Important And Useful

for most of the studies of these galaxies (Górski et al. ApJ, 889, 179). - also in 2020 an important advance on the empirical calibration of the SBCR based on interferometric data was presented by Nardetto et al. (2020, A&A, 639, A67).

- also in 2020 our group published a 2% geometric dis­ tance to the SMC from late type eclipsing binary sys­ tems (Graczyk et al. 2020, ApJ, 904, 13). This work im­ proved on our 2014 result and concluded our 15-year- long project, which was one of the most important ones in the Araucaria Project. It has already become the stan­ dard reference for the SMC distance in the literature.

- in 2021 we continued studies on binarity of Cepheids and RR Lyrae stars. Pilecki et al. 2021 (ApJ, 910, 118)

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detected a numerous population of double-lined bina­ ries in the LMC hosting a Cepheid component; Hajdu et al. (ApJ, 915, 50) presented a very detailed study of binaries containing RR Lyrae stars as detected from a study of their period variability.

- also in 2021, thanks to our geometrical distances and Gaia DR3 parallaxes another precise determination of the Cepheid metallicity effect was derived and pub­ lished (Breuval et al. ApJ, 913, 38). The results are in excellent agreement with those found in a complete­ ly independent way by our group before (Gieren et al.

2018, A&A, 620, A99).

- also in 2021 the first calibration of the SBCR from eclipsing binaries and Gaia DR3 parallaxes was pub­ lished (Graczyk et al. A&A, 649, A109). The results show the great potential of this approach.

The Araucaria Project Team Members Held Several

internal workshops over the years (sometimes enriched by external collaborators) which proved to provide excellent opportunities for brainstorming, and social events which helped to strengthen the spirit of friend­ ship among our group, including the ever-increasing number of graduate students who became part of the project over the years.

Fig. 7. Araucaria Meeting in Sopot, Poland, in 2021, twenty-one years after founding.

Candles: Classical Cepheids, Blue Supergiants,

tip of the red giant branch stars, eclipsing binaries, and more. It is a generously equipped cosmic laboratory, where one can study a lot: the metallicity gradient in the disc from the spectroscopic analysis of blue supergi­ ants (Bresolin et al. 2002), the effect of metallicity on the intrinsic brightness of classical Cepheids, but above all, the distances determined independently from different standard candles, especially from classical Cepheids (hereafter just Cepheids). The only problem before the year 2002 was that NGC 300 was virtually uncharted territory, and the reported population of classical Ce­

Pheids Was Only 18 (Graham 1984). Why Has Ngc 300

not been studied thoroughly? A few reasons stacked up. NGC 300 is a large galaxy in terms of angular size (19’ x 13’), and covering such an area on the sky required a large field camera, which before the year 1999 simply hadn’t existed. In 1999 the Wide Field Imager (WFI, FoV: 34’ x 33’) on MPG/ESO 2.2-m telescope at the La Silla Ob­ servatory, Chile, was commissioned, making the obser­ vations of spatially extended objects feasible and very time-efficient. The enormous images produced by the WFI required dedicated time and human resources to be properly processed and analyzed, which meant that Paulina Karczmarek1, Wolfgang Gieren1, Grzegorz Pietrzyński2

Optical Survey Of Classical Cepheids

1 Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile 2 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland of the Universidad de Concepción was announced.

A resourceful and ingenious Polish astronomer, Grze­ gorz Pietrzyński, joined the team of Wolfgang Gieren, and they embarked on a scientific journey to find more Cepheids in NGC 300.

Prompted By The Fact That Ngc 300 Shows Signs Of

recent, massive star formation, and therefore should host a significant number of Cepheids, Wolfgang ap­ plied for observing time at ESO and was awarded the generous number of 29 nights between July 1999 and January 2000. During this time around 150 images of NGC 300 in each of B, V, I bands were collected. From around 32,000 observed stars Grzegorz selected targets of specific color index (0.4 mag < (B-V) < 1.5 mag), that showed the asymmetrical sawtooth light curves typi­ cal for classical Cepheids, and that had larger pulsa­ tion amplitudes in the B-band than in the V-band, with

The B-Band Amplitude Exceeding 0.4 Mag. Crawling

through a database of tens of thousands objects took

Some Time But Every Now And Then, A Cepheid Would

pop out! Soon not only had we recovered all previous­

Ly Known 18 Cepheids, But We Increased The Known

sample by a factor of six, resulting in a collection of 117 Cepheids (Pietrzyński et al. 2002). The Araucaria Project began from, and gained momentum with, a study of a graceful galaxy in the Sculptor Group, NGC 300. It is a spiral, near face-on galaxy, with a metallicity gradient ranging from solar-like in the center to SMC-like on the peripheries.

The Discovered Cepheids Span A Broad Range Of

pulsation periods, from 115 to 5.4 days, with a period uncertainty of only 0.005-0.01 days, thanks to greatly extended time baseline since the first epoch observa­ tions (Graham 1984), which now corresponds to about 76-1000 pulsation cycles. The high accuracy of our data is reflected by the uncertainties of the photometric zero points, which in both B- and V-bands are with­ in 0.03 mag, and agree very well with those present­ ed by Freedman et al. (2002). The completeness of our sample drops suddenly for objects fainter than about 22.5 mag in the V-band, corresponding to a pulsation period of around 10 days. In other words, our survey is virtually complete for Cepheids with periods longer than 10 days. The spatial distribution of Cepheids, that shows how they trace the spiral arms of the galaxy, can be appreciated in Figure 1, together with finding charts and light curves of five exemplary Cepheids.

Of Five Exemplary Cepheids. Figure Assem­

bled from figures 1, 3, 5 of Pietrzyński et al. (2002).

20

Figure 2 shows that all 117 Cepheids follow the pe­ riod-luminosity relation in the B- and V-band, howev­ er the spread is too large to accurately determine the distance to NGC 300 from the visual passbands alone.

A much more accurate distance determination can be achieved in the near-infrared (NIR) domain, where the Cepheids follow the period-luminosity relation more tightly and are less affected by the reddening. The idea of carrying out NIR observations has been integrated into

A Large Programme That We Applied For At Eso In

2003. A survey in the visual passbands was necessary to find Cepheids by their large amplitude, asymmet­ rical light curves, and pick the best sample for a fol­

Distance And Reddening Simultaneously, And Derive

a true, dereddened distance of superior accuracy (Gie­ ren et al. 2005 and in this volume).

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