Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations
In 2011 a promising candidate for an RR Lyrae star in an eclipsing binary system was found. Till that time not even one case of RR Lyrae star in a binary system has been known. The pulsator's mass is 0.26 Mꙩ which is not enough to burn helium in the core, as RR Lyrae stars do. The presence of...
Збережено в:
| Дата: | 2012 |
|---|---|
| Автор: | |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Головна астрономічна обсерваторія НАН України
2012
|
| Назва видання: | Advances in Astronomy and Space Physics |
| Онлайн доступ: | http://dspace.nbuv.gov.ua/handle/123456789/119184 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations / P. Karczmarek // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 2. — С. 135-138. — Бібліогр.: 10 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-119184 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1191842017-06-05T03:04:02Z Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations Karczmarek, P. In 2011 a promising candidate for an RR Lyrae star in an eclipsing binary system was found. Till that time not even one case of RR Lyrae star in a binary system has been known. The pulsator's mass is 0.26 Mꙩ which is not enough to burn helium in the core, as RR Lyrae stars do. The presence of a more massive companion is a clue that the mass transfer had to occur in the past. Therefore, Binary Evolution Pulsating (BEP) star, while having RR Lyr-like light curve, has completely unlike internal structure. The bulk of the star's mass was lost during the red giant phase due to mass transfer and the partially degenerated helium core with thin hydrogen burning shell was revealed. The BEP object has been captured inside the instability strip (IS) in the RR Lyrae area and thus it is confused with classical RR Lyrae pulsators. Therefore, the BEP star is the evidence of a new evolutionary channel to produce RR Lyr-like oscillations. In simulations made with StarTrack code we trace the evolution of a sample of binaries and examine properties of the system required for pulsation phase to occur. We suggest that the stars created via this new evolutionary channel can in part explain the existence of UV up-turn, low-mass C-O WD and He WD. 2012 Article Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations / P. Karczmarek // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 2. — С. 135-138. — Бібліогр.: 10 назв. — англ. 2227-1481 http://dspace.nbuv.gov.ua/handle/123456789/119184 en Advances in Astronomy and Space Physics Головна астрономічна обсерваторія НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
In 2011 a promising candidate for an RR Lyrae star in an eclipsing binary system was found. Till that time not even one case of RR Lyrae star in a binary system has been known. The pulsator's mass is 0.26 Mꙩ which is not enough to burn helium in the core, as RR Lyrae stars do. The presence of a more massive companion is a clue that the mass transfer had to occur in the past. Therefore, Binary Evolution Pulsating (BEP) star, while having RR Lyr-like light curve, has completely unlike internal structure. The bulk of the star's mass was lost during the red giant phase due to mass transfer and the partially degenerated helium core with thin hydrogen burning shell was revealed. The BEP object has been captured inside the instability strip (IS) in the RR Lyrae area and thus it is confused with classical RR Lyrae pulsators. Therefore, the BEP star is the evidence of a new evolutionary channel to produce RR Lyr-like oscillations. In simulations made with StarTrack code we trace the evolution of a sample of binaries and examine properties of the system required for pulsation phase to occur. We suggest that the stars created via this new evolutionary channel can in part explain the existence of UV up-turn, low-mass C-O WD and He WD. |
| format |
Article |
| author |
Karczmarek, P. |
| spellingShingle |
Karczmarek, P. Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations Advances in Astronomy and Space Physics |
| author_facet |
Karczmarek, P. |
| author_sort |
Karczmarek, P. |
| title |
Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations |
| title_short |
Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations |
| title_full |
Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations |
| title_fullStr |
Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations |
| title_full_unstemmed |
Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations |
| title_sort |
binary evolution pulsating star - new evolutionary channel to produce rr lyr-like pulsations |
| publisher |
Головна астрономічна обсерваторія НАН України |
| publishDate |
2012 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/119184 |
| citation_txt |
Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations / P. Karczmarek // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 2. — С. 135-138. — Бібліогр.: 10 назв. — англ. |
| series |
Advances in Astronomy and Space Physics |
| work_keys_str_mv |
AT karczmarekp binaryevolutionpulsatingstarnewevolutionarychanneltoproducerrlyrlikepulsations |
| first_indexed |
2025-07-08T15:23:16Z |
| last_indexed |
2025-07-08T15:23:16Z |
| _version_ |
1837092775370162176 |
| fulltext |
Binary evolution pulsating star � new evolutionary channel
to produce RR Lyr-like pulsations
P.Karczmarek
∗
Advances in Astronomy and Space Physics, 2, 135-138 (2012)
© P.Karczmarek, 2012
Centre for Astronomy, Nicolaus Copernicus University, 11 Gagarina st., 86-100 Toru«, Poland
In 2011 a promising candidate for an RR Lyrae star in an eclipsing binary system was found. Till that time not
even one case of RR Lyrae star in a binary system has been known. The pulsator's mass is 0.26M� which is not
enough to burn helium in the core, as RR Lyrae stars do. The presence of a more massive companion is a clue that
the mass transfer had to occur in the past. Therefore, Binary Evolution Pulsating (BEP) star, while having RR
Lyr-like light curve, has completely unlike internal structure. The bulk of the star's mass was lost during the red
giant phase due to mass transfer and the partially degenerated helium core with thin hydrogen burning shell was
revealed. The BEP object has been captured inside the instability strip (IS) in the RR Lyrae area and thus it is
confused with classical RR Lyrae pulsators. Therefore, the BEP star is the evidence of a new evolutionary channel
to produce RR Lyr-like oscillations. In simulations made with StarTrack code we trace the evolution of a sample
of binaries and examine properties of the system required for pulsation phase to occur. We suggest that the stars
created via this new evolutionary channel can in part explain the existence of UV up-turn, low-mass C-O WD and
He WD.
Key words: stars: variables: RR Lyrae � binaries: eclipsing � stars: evolution
introduction
Eclipsing binaries are perfect stellar laboratories
to determine the most important stellar parame-
ters: age, mass, radius, metallicity. The light curve
and radial velocities analysis of binary components
now provide the stellar parameters with the accu-
racy of 1% [1]. Moreover, thanks to pulsating stars
in eclipsing binaries, the problem of dynamical mass
of the classical Cepheids was solved � the result was
in a better agreement with the predictions of the
stellar pulsation theory than the stellar evolution
one [6]. Determination of the parameters of other
types of eclipsing pulsators, as δ Scuti stars or clas-
sical Cepheids, improves the understanding of stel-
lar physics and imposes the constraints on the key
stellar parameters. In case of standard candles, it al-
lows for a better calibration of the cosmic distances.
The growing number of discoveries of the eclipsing
pulsating stars is being reported nowadays, yet no
RRLyrae star has been found.
the discovery
In 2011, Soszy«ski et al. [8] observed an eclips-
ing binary with the RR Lyrae component, named
OGLE-BLG-RRLYR-02792, with an orbital period
Porb = 15.24 d and the pulsation period Ppul =
0.627d. The method used to untangle the pulsa-
tion and eclipse curves is based on �tting a func-
tion to the data via the Fourier series. The program
designed for this task �nds the best �t of the light
curve phased with pulsational period (with already
removed �eclipse points�) to the data, subtracts the
newly-found function from the light curve which con-
tains all points (�eclipse points� present). Subtrac-
tion eliminates the pulsation variability and retains
only eclipses.
Fig. 1: Pulsational I-band light curve of the primary
component of the binary system OGLE-BLG-RRLYR-
02792, folded on a pulsation period of 0.627548 days.
The shape of the light curve is mimicking that of a clas-
sical RRLyr star. The outlier points are responsible for
the eclipsing variability. The �gure excerpted from [8].
∗paulina.karczmarek@astri.uni.torun.pl
135
Advances in Astronomy and Space Physics P.Karczmarek
Fig. 2: Orbital I-band light curve (617 epochs col-
lected over 10 years) of the binary system OGLE-BLG-
RRLYR-02792, after removal of the intrinsic brightness
variation of the pulsating component (data points), to-
gether with the solution (solid line), as obtained with
the 2007 version of the standard Wilson-Devinney code
[9, 10]. Top panel: the residuals of the observed magni-
tudes from the computed orbital light curve. The �gure
excerpted from [7].
Table 1: Orbital and physical parameters of the OGLE-
BLG-RRLYR-02792 system, together with their uncer-
tainties as obtained from the modelling of the spectro-
scopic and photometric data. The table excerpted from
[7].
Parametera Primary (pulsating) Secondary
M [M�] 0.261 ± 0.015 1.67 ± 0.06
R [R�] 4.24 ± 0.24 4.27 ± 0.31
Teff [K] 7320 ± 160 5000 ± 150
a [R�] 32.20 ± 0.32
Porb [d] 15.24350 ± 0.00021
Ppul [d] 0.627548 ± 0.000008
dP/dt −2.3× 10−8
e 0.0072± 0.0029
a The parameters are as follows: stellar mass, stel-
lar radius, e�ective temperature, orbit size, orbital
period, pulsational period, rate of period change, ec-
centricity.
Figs. 1 and 2 give the general view on the star's
variability. Collected data was carefully analysed by
Pietrzy«ski et al. [7] to determine the orbital and
physical parameters of the system (Table 1) and the
type of pulsations using the Fourier parameters. All
indicators: the Fourier parameters, the pulsational
period and the location on Hertzsprung-Russel (HR)
diagram, implied with no doubts that the pulsating
component is RRLyrae star but the pulsator's mass
was not enough (0.26M�) for the star to undergo the
helium �ash and thus to burn helium in the core, as
it is usually takes place in canonical RRLyrae stars.
The presence of the companion could explain the
troublesome mass of the pulsating component. In
the past the system has undergone the mass trans-
fer episode and as a result the donor has stripped
its almost entire hydrogen envelope o� revealing the
hot helium core. The physical properties of the pul-
sator happen to place it in the same instability strip
(IS) of the HR diagram occupied by RRLyrae stars.
It is noteworthy that the pulsations were generated
not as a result of the single star evolution but due
to the evolution of the binary system, therefore the
new type of binary star gained the name Binary Evo-
lution Pulsator (BEP). BEPs are considered to pass
the IS two orders of magnitude faster than canonical
RRLyrae stars, and always towards higher temper-
atures.
simulations
The simulations made with the StarTrack code
(manual to the code is available in [2]) enables to
track the evolution of the binary from Zero Age Main
Sequence (ZAMS) till the white dwarf (WD) phase.
If at any time of the evolution, any of two com-
ponents crosses the RRLyrae instability strip while
having the mass M < 0.5M�, this object at this
particular stage of its evolution is called BEP and
initial parameters of the binary system (masses of
both components, orbital period) are collected. The
RRLyrae instability area is determined in the terms
of e�ective temperatures (Teff) and luminosities (L)
as follows [3]:
16L� < L < 100L�
5000K < Teff < 7400K. (1)
Two di�erent kinds of BEP objects crossing the IS
were found:
(i) M . 0.3M�. The primary component starts
its evolution as a main sequence (MS) star and ends
as He WD after 5-7Gyrs. While being red giant
(RG) it transfers the mass to the companion (MS
star). After the mass transfer ended, primary com-
ponent enters the Horizontal Branch without going
through the helium �ash. The luminosity remains
the same as the radius decreases and the tempera-
ture increases. The primary component pulsates as
BEP while crossing the IS in the time interval of
about a million years. The secondary component
goes through the RG phase, expands and over�lls its
Roche Lobe. The unstable mass transfer to the pri-
mary leads to the common envelope episode. At the
�nal stage the binary consists of two helium white
dwarfs. Fig. 3 shows the evolutionary track of the
primary.
(ii) 0.3M� . M . 0.5M�. The primary compo-
nent starts its evolution as a MS star and ends as C-O
WD after 1.5-2.5Gyrs. While being RG it transfers
136
Advances in Astronomy and Space Physics P.Karczmarek
the mass to the companion (MS star) until reaches
the tip of the RG branch and ignites helium in the
core. Then the mass transfer ends and the primary
descend the RG branch and resides in the Horizon-
tal Branch. The luminosity remains the same as the
radius decreases and the temperature increases. The
primary pulsates as BEP while crossing the IS in the
time interval of about a million years. The secondary
component goes through the Red Giant phase, ex-
pands and over�lls its Roche Lobe. The unstable
mass transfer to the primary leads to the common
envelope episode. At the �nal stage the binary con-
sists of two C-O WD or the C-O and hybrid WD.
Fig. 4 shows the evolutionary track of the primary.
The �rst (i) �low-mass� case is available for initial
parameters: the mass of the primary and the orbital
period, in ranges
0.4 M� . M0 . 2.0 M�
1.4 d . P0 . 9.0 d, (2)
while the (ii) �high-mass� case occurs when
2.0 M� . M0 . 3.0 M�
4.0 d . P0 . 18.0 d. (3)
The constraints are �uent and require further exam-
inations to be estimated more accurate.
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
3.54.04.55.0
lo
g
(L
/L
❍•
)
log(Teff) [K]
M0A = 1.4 M0B = 0.8
P0 = 2.9
Fig. 3: Evolutionary track of the primary component
which undergoes the BEP phase having the mass M .
0.3M�. Thick line on the evolutionary path indicates
the mass transfer phase. The trapezium shaped area is
the IS of RRLyrae stars. The initial masses and period
of the binary are given.
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
3.54.04.55.0
lo
g
(L
/L
❍•
)
log(Teff) [K]
He
M0A = 2.0 M0B = 1.6
P0 = 12.9
Fig. 4: Evolutionary track of the primary compo-
nent which undergoes the BEP phase having the mass
0.3M� & M & 0.5M�. Thick line on the evolution-
ary path indicates the mass transfer phase. Dashed line
shows the helium core burning stage which starts on the
tip of the RG branch just after the mass transfer episode
ends. The trapezium shaped area is the IS of RRLyrae
stars. The initial masses and period of the binary are
given.
results and conclusions
Long time-base observations of stars inside the
IS allow to determine the rate of pulsation pe-
riod change dP/dt. Because the star is moving
at a constant luminosity across the IS towards
higher (lower) temperatures, its radius should be-
come smaller (larger) and therefore its pulsation pe-
riod should steadily decrease (increase). For clas-
sical RRLyrae stars |dP/dtRR| ≈ 10−11 − 10−9 (in
alternative notation 0.01 − 1.0 d/Myr) [5] and can
be positive or negative as they cross IS two times,
�rst going towards lower temperatures, next � to-
wards higher temperatures. BEPs, in turn, cross the
IS only once, always towards higher temperatures,
that means dP/dtBEP < 0 and as they are moving
rapidly, −2.2 × 10−8 . dP/dtBEP . −5.5 × 10−9
(equals to dP/dtBEP ≈ 2 − 8 d/Myr). This dis-
tinct di�erence can be used to distinguish BEPs from
RRLyrae stars, especially in case when the eclipses
are not seen in a system.
BEP is just a part of life of the object which there-
after turns into He WD or C-O WD (depending on
the initial mass). Such evolution can in part explain
the abundance of He WD and low-mass C-O WD.
The single star simulations show that these objects
137
Advances in Astronomy and Space Physics P.Karczmarek
can be created from the red giant provided arti�cially
high mass loss [4]. Simulations with StarTrack code
performed for the binary case show that the high
mass loss rate is fully explained by the mass transfer
that happens exactly in the RG phase. As the result,
the hot (helium or carbon) core stripped from hy-
drogen envelope tends to radiate in short-wavelength
spectrum band. To sum up, the binary scenario ac-
counts for existence of He WD, C-O WD and UV
up-turn.
The contamination of RR Lyrae stars by BEPs
was estimated to be 0.2% [7]. This means that for
1000 objects classi�ed as RRLyrae stars, two can
actually be BEPs, only showing the RR Lyr-like
light curve. Nevertheless, detailed calculations are
required to con�rm this number. The higher percent
of contamination might increase the observed spread
in luminosity of the RRLyrae stars and a�ect dis-
tance measurements based on them. As the BEPs
are much younger than the RRLyrae stars, the age
of old galaxies and globular clusters hosting them
might be rede�ned as younger. It is also possible
that the contamination by BEPs may concern not
only the RR Lyrae stars but also the other pulsators
in the IS, like classical Cepheids or δ Scuti stars.
The discovery of OGLE-BLG-RRLYR-02792
points to the new evolutionary channel to create
RRLyr-like oscillations. This encourages to improve
both evolutionary and pulsation theories. The pri-
ority task is to proceed tracking pulsating binaries
and to double check the stellar catalogues which can
already contain the �fake� RRLyrae stars and other
binaries. Finally, it is a chance to enrich the evo-
lutionary and pulsation codes with new aspects of
binary evolution and to calibrate them for better pre-
dictions of stars' behaviour.
acknowledgement
It is a pleasure to thank G.Pietrzy«ski for sup-
porting this project. I am grateful to K.Belczy«ski
for sharing the StarTrack code and for many useful
and valuable instructions. The research presented
here was supported by the TEAM subsidies of the
Foundation for Polish Science (FNP).
references
[1] Andersen J., Clausen J.V., NordstromB., Tomkin J. &
MayorM. 1991, A&A, 246, 99
[2] BelczynskiK., KalogeraV. & BulikT. 2002, ApJ, 572,
407
[3] BonoG., CaputoF., Cassisi S., IncerpiR. & MarconiM.
1997, ApJ, 483, 811
[4] CastellaniM., CastellaniV. & Prada Moroni P.G. 2006,
A&A, 457, 569
[5] KunderA., WalkerA. S., PeterB. et al. 2011, AJ, 141,
15
[6] Pietrzy«skiG., Thompson I. B., GierenW. et al. 2010,
Nature, 468, 542
[7] Pietrzy«skiG., Thompson I. B., GierenW. et al. 2012,
Nature, 484, 75 2
[8] Soszy«ski I., DziembowskiW.A., UdalskiA. et al. 2011,
Acta Astronomica, 61, 1
[9] Van HammeW. & WilsonR.E. 2007, ApJ, 661, 1129
[10] WilsonR.E. & DevinneyE. J. 1971, ApJ, 166, 605
138
|