J/A+A/690/A36 TIC 441725813 spectrum and prewhitened frequencies (Su+, 2024)
TIC 441725813: A new bright hybrid B subdwarf pulsator with differential
core/envelope rotation.
Su W., Charpinet S., Latour M., Zong W., Green E.M., Li G.
<Astron. Astrophys. 690, A36 (2024)>
=2024A&A...690A..36S 2024A&A...690A..36S (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Spectra, optical ; Asteroseismology
Keywords: binaries: close - stars: oscillations - stars: rotation
Abstract:
The Transiting Exoplanet Survey Satellite (TESS) performs
high-precision photometry over almost the whole sky primarily in
search of exoplanet transits. It also provides exquisite data to study
stellar variability, in particular for pulsating hot B subdwarf (sdB)
stars.
We present the detailed analysis of a new hybrid (p- and g-mode) sdB
pulsator, TIC 441725813 (TYC 4427-1021-1), discovered and monitored by
TESS over 670 days.
The TESS light curves available for this star were analyzed using
prewhitening techniques to extract mode frequencies accurately. The
pulsation spectrum was then interpreted through methods that include
asymptotic period spacing relationships and the identification of
rotational multiplets. We also exploited a high signal-to-noise ratio
(S/N), low-resolution spectrum of TIC 441725813 using grids of
non-local thermodynamic equilibrium (NLTE) model atmospheres to derive
its atmospheric parameters.
The light curve analysis reveals that frequencies are mostly found in
the g-mode region, but several p-modes are also detected, indicating
that TIC 441725813 is a hybrid sdB pulsator. We identify 25
frequencies that can be associated with l=1 g-modes, 15 frequencies
corresponding to l=2 g-modes, and six frequencies characteristic of
p-modes. Interestingly, several frequency multiplets interpreted as
rotational splittings of deep-probing g-modes indicate a slow rotation
period of at least 85.3±3.6 day, while splittings of mostly
envelope-probing p-modes suggest a significantly shorter rotation
period of 17.9±0.7 day, which implies the core (mainly the helium
mantle with possibly the deeper partially mixed helium-burning core
that it surrounds) rotates at least 4.7 times slower than the
envelope. The radial velocity curves indicate that TIC 441725813 is in
a close binary system with a low-luminosity companion, possibly a
white dwarf. While elusive in the available TESS photometry, a
low-frequency signal that would correspond to a period of ∼6.7 h is
found, albeit at a low S/N. Furthermore, we estimate the inclination
angle to be ∼60° by two independent means.
TIC 441725813 is a particularly interesting sdB star whose envelope
rotates faster than the core. We hypothesise that this might be caused
by the effects of a tidal interaction with a companion, although in
the present case, the presence of such a companion will have to be
further investigated. This analysis paves the way towards a more
detailed seismic probing of TIC 441725813 using optimisation
techniques, which will be presented in a second paper.
Description:
Taking into account time gaps and available cadences of TESS
observations, we divided the data into four date sets for independent
analyses. The first data set (g1) includes 12 consecutive sectors (S14
to S25), in which TESS has continuously monitored TIC 441725813 with
the 2-min cadence mode for more than 300 days. The second data set
(g2) consists of S40 and S41 which cover about 55 days. Short-cadence
observations (20s mode) became available at that time and could reveal
higher-frequency signals. Therefore, we chose to use the 20s data when
both cadences were available. The remaining 12 sectors (S47 to S52,
S55 to S60) make up the third (g3) and fourth (g4) data sets, each
with a total time baseline of more than 150 days. We performed
standard prewhitening and nonlinear least-square fitting techniques to
extract all frequencies with S/N above 5.2 times the local median
noise. All prewhitened frequencies in each data sets are shown in the
table, including periods and S/N.
The star was observed five times with the 2.3m Bok telescope at
Steward Observatory on Kitt Peak between September 2005 and July 2008.
The observations were obtained with the B&C spectrograph using the
400mm-1 grating in first order and a 2.5-arcsec slit, resulting in a
wide wavelength coverage (3620-6900Å) and a rather low resolution
of 8.7Å. The final spectrum with high S/N was constructed from the
median-filtered individual spectra that were first cross-correlated to
correct for the variations in radial velocity.
Objects:
-------------------------------------------------------------
RA (2000) DE Designation(s)
-------------------------------------------------------------
17 04 58.37 +73 04 43.2 TIC 441725813 = TYC 4427-1021-1
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
allfre.dat 170 284 Raw (unfiltered) list of all prewhitened
frequencies in 4 data sets available for
TIC 441725813
list.dat 99 1 Information on spectrum file
fits/* . 1 TIC 441725813 reduced spectrum
--------------------------------------------------------------------------------
Byte-by-byte Description of file: allfre.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 A6 --- Freq Frequency ID
9- 17 F9.4 uHz g1freq ?=- Effective frequency extracted
from dataset g1 (1)
19- 24 F6.4 uHz e_g1freq ? Effective frequency extracted
from dataset g1 error
26- 34 F9.3 uHz g2freq ?=- Effective frequency extracted
from dataset g2 (2)
36- 40 F5.3 uHz e_g2freq ? Effective frequency extracted
from dataset g2 error
42- 50 F9.4 uHz g3freq ?=- Effective frequency extracted
from dataset g3 (3)
52- 57 F6.4 uHz e_g3freq ? Effective frequency extracted
from dataset g3 error
59- 67 F9.4 uHz g4freq ?=- Effective frequency extracted
from dataset g4 (4)
69- 74 F6.4 uHz e_g4freq ? Effective frequency extracted
from dataset g4 error
76- 85 F10.4 s g1period ?=- Period corresponding to the frequency
in g1 (1)
87- 92 F6.4 s e_g1period ? Period corresponding to the frequency
in g1 error
94-102 F9.3 s g2period ?=- Period corresponding to the frequency
in g2 (2)
104-108 F5.3 s e_g2period ? Period corresponding to the frequency
in g2 error
110-119 F10.4 s g3period ?=- Period corresponding to the frequency
in g3 (3)
121-126 F6.4 s e_g3period ? Period corresponding to the frequency
in g3 error
128-137 F10.4 s g4period ?=- Period corresponding to the frequency
in g4 (4)
139-144 F6.4 s e_g4period ? Period corresponding to the frequency
in g4 error
146-150 F5.1 --- g1S/N ?=- S/N corresponding to the frequency in g1
152-157 F6.2 --- g2S/N ?=- S/N corresponding to the frequency in g2
159-164 F6.2 --- g3S/N ?=- S/N corresponding to the frequency in g3
166-170 F5.1 --- g4S/N ?=- S/N corresponding to the frequency in g4
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Note (1): For g1, Amplitude = 0.96*10-3*S/N (%)
Note (2): For g2, Amplitude = 1.95*10-3*S/N (%)
Note (3): For g3, Amplitude = 1.48*10-3*S/N (%)
Note (4): For g4, Amplitude = 1.74*10-3*S/N (%)
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Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 34 A10 "date" Obs.date Observation date
36- 39 I4 0.1nm bWave Lower value of wavelength interval
41- 44 I4 0.1nm BWave Upper value of wavelength interval
46- 52 F7.5 0.1nm dWave Wavelength resolution
54- 55 I2 Kibyte size Size of FITS file
57- 68 A12 --- FileName Name of FITS file, in subdirectory fits
70- 99 A30 --- Title Title of the FITS file
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Acknowledgements:
Wenchao Su, wenchao.su(at)irap.omp.eu
(End) Wenchao Su [IRAP, France], Patricia Vannier [CDS] 23-Jul-2024