Issue |
A&A
Volume 516, June-July 2010
|
|
---|---|---|
Article Number | L6 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014789 | |
Published online | 22 June 2010 |
LETTER TO THE EDITOR
CoRoT opens a new era in hot B subdwarf asteroseismology
,![[*]](/icons/foot_motif.png)
Detection of multiple g-mode oscillations in KPD 0629-0016
S. Charpinet1 - E. M. Green2 - A. Baglin4 - V. van Grootel1 - G. Fontaine3 - G. Vauclair1 - S. Chaintreuil4 - W. W. Weiss5 - E. Michel4 - M. Auvergne4 - C. Catala4 - R. Samadi4 - F. Baudin6
1 - Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse,
CNRS, 14 Av. E. Belin, 31400 Toulouse, France
2 -
Steward
Observatory, University of Arizona, 933 North Cherry Avenue, Tucson,
AZ 85721, USA
3 - Département de Physique,
Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal,
QC H3C 3J7, Canada
4 - LESIA, Observatoire de
Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
5 - Institute
for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
6 - Institut d'Astrophysique Spatiale (IAS), bâtiment 121,
91405 Orsay Cedex, France
Received 14 April 2010 / Accepted 25 May 2010
Abstract
Context. The asteroseismic exploitation of long period, g-mode
hot B subdwarf (sdB) pulsators has been a long sought objective
undermined, thus far, by the difficulty of obtaining sufficiently
precise and continuous time series data from the ground.
Aims. Fast photometry from space appears to be the only means of
gathering the appropriate asteroseismic data for this type of star. We
explore this possibility with the C OR OT (COnvection, ROtation, and planetary Transits) satellite.
Methods. We obtained 24 days
of high quality, nearly continuous photometric data with CoRoT during a
short run (SRa03) dedicated to the long period sdB pulsator
KPD 0629-0016. We analysed the frequency (period) content of the C
OR OT time series by combining Fourier analysis, nonlinear least squares fitting, and prewhitening techniques.
Results. Our study has led to the detection of a large number of g-mode
pulsations in KPD 0629-0016, with 17 frequencies clearly
identified in addition to 7 possible (although more
uncertain) peaks emerging above the mean noise level (estimated at 57 ppm).
This is more than is typically detected for sdB stars from the ground
and, more importantly, the frequencies of all uncovered g-modes
are, for the first time, reliably measured. This paves the way for a
detailed asteroseismic analysis of this star. The oscillations are
found in the 90-400
Hz frequency range with a dominant mode at 205.29
Hz (P=1.353 h) of amplitude 0.246% of the mean brightness, i.e., typical of mid-radial order g-mode pulsations.
Conclusions. These photometric observations of KPD 0629-0016 demonstrate that g-mode sdB pulsators have rich oscillation spectra that are accessible to current space-based facilities. C OR OT opens up a new era in asteroseismology of hot B subdwarf stars.
Key words: stars: oscillations - subdwarfs - stars: individual: KPD 0629-0016
1 Introduction
The occurrence of nonradial pulsations in some hot B subdwarf (sdB)
stars provides an extraordinary tool for probing, by asteroseismic
means, the inner structure and dynamics of stars that are representative of
an intermediate stage of stellar evolution. SdB stars are associated
with the so-called extreme horizontal branch (EHB) corresponding to
low-mass (0.5
)
objects burning helium in their
core. They differ from classical horizontal branch stars mainly in terms of
their residual H-rich envelope, which has been stripped
down dramatically during the previous stage of evolution, leaving
only a very thin layer less massive than 0.02
.
For this
reason, sdB stars are hot (
K)
and compact (
), and never ascend the asymptotic
giant branch before fading as white dwarfs. It is unclear which
mechanisms determine whether a star evolving through the red giant phase
eventually loses (or not) all but a tiny fraction of its envelope.
Close binary evolution through various channels is probably the main
source of sdB stars (Han et al. 2003,2002),
but isolated main sequence progenitors passing through the red giant
branch and experiencing, for some reason, enhanced mass loss cannot
be totally excluded (D'Cruz et al. 1996).
Two classes of sdB pulsators provide the ``basic material'' for
developing asteroseismology as a new investigation tool for this intermediate
evolutionary stage. The sdB
(or V361 Hya, or EC 14026; Kilkenny et al. 1997)
stars oscillate rapidly with periods in the 100-600 s range,
corresponding mostly to low-order, low-degree p-modes. These modes
are driven by a classical
-mechanism occurring in the zone
of partial ionization of the iron-group elements (the Z-bump) and
powered-up by radiative levitation (Charpinet et al. 1997,1996).
The sdB
(or V1093 Her; Green et al. 2003) stars pulsate far
more slowly with periods of
1-2 h, corresponding to mid-order
gravity modes. The same mechanism drives these oscillations (Fontaine et al. 2003).
A few stars belong to both classes and are called hybrid (or sdB
)
pulsators,
showing both p- and g-modes (e.g., Schuh et al. 2006;
see also the review by Charpinet et al. 2009).
Thus far, a dozen asteroseismic inferences have been successfully
obtained from pulsating hot subdwarf stars (e.g.,
van Grootel et al. 2008a; Charpinet et al. 2009,2008; van Grootel et al. 2008b; Randall et al. 2009),
but all were short period p-mode pulsators.
The rapid oscillations associated with relatively large amplitudes
(
of the mean brightness, but up to
in some cases)
have indeed favoured
these pulsators as preferred targets for seismic analysis based on
data obtained from the ground. However, with the discovery of long
period sdB pulsators, hope has grown that these could reveal the structure
of the deepest regions in stars of this type, including the thermonuclear
furnace. This is because g-modes in hot B subdwarf stars propagate
toward the deep core, while p-modes remain confined to the outermost
layers (Charpinet et al. 2000). However, despite valiant efforts
from the ground (Randall et al. 2006b; Baran et al. 2009; Randall et al. 2006a), it has been extremely difficult to
differentiate the pulsation frequencies from the many aliases
introduced by the lack of continuous coverage, particularly because
of the much longer periods and the very
low amplitudes (typically
)
involved. The asteroseismic study of
g-mode sdB pulsators clearly requires accurate and continuous photometry in
orbit.
The first attempt to observe a pulsating hot B subdwarf star
from space was the M OST satellite monitoring for 400 h of the long period sdB star PG 0101+039 (Randall et al. 2005).
While pulsations in this star were effectively detected based on these data,
the very low amplitudes and the relative faintness
of the target allowed the detection of only 3 frequencies, which are insufficient
for a thorough asteroseismic study. This pioneering effort, however,
left great hope that other stellar photometry satellites such as C OR OT
(Baglin et al. 2006) and Kepler (Gilliland et al. 2010)
would provide the needed breakthrough in this domain.
![]() |
Figure 1: Location of KPD 0629-0016 (black open circle) in the effective temperature-surface gravity plane in relation to the other known pulsating hot B subdwarf stars using a sample with reliable and homogeneous spectroscopic measurements. The short-period p-mode pulsators (V361 Hya stars) are shown in blue, while the long-period g-mode pulsators (V1093 Her stars) are shown in red. The three known hybrid stars showing simultaneously both p-mode and g-mode oscillations are also clearly indicated at the commun boundary of both classes. KPD 0629-0016 belongs to the V1093 Her family. |
Open with DEXTER |




2 Time series photometry and frequency content
![]() |
Figure 2: C OR OT light curve obtained for KPD 0629-0016 during SRa03 observations. The amplitude as a function of time is expressed in terms of the residual intensity relative to the mean brightness of the star. The red curve in the bottom panel shows the reconstructed signal based on the extracted frequencies, amplitudes, and phases given in Table 1. |
Open with DEXTER |
The hot B subdwarf KPD 0629-0016 was observed by CoRoT over a time
baseline of
days (
558.96 h) from March 5 to 29, 2010 (the scheduled SRa03 run). Because this star is relatively
faint (V=14.91), it was positioned on a CCD dedicated to the
planet finding program, which was designed to perform an accurate simultaneous
photometric monitoring of thousands of stars in the 12-16 mag range.
Every 32 s, except during a short interruption of
3.55 h
at the end of day 13, a small image of KPD 0629-0016 was acquired. Basic
reduction to remove known instrumental effects was performed by applying the
standard reduction pipeline that produces exploitable light curves (see, e.g.,
Auvergne et al. 2009).
The ``white light'' photometry obtained for KPD 0629-0016 reaches an effective
duty cycle of
80.5%, leaving side lobes in the window function that
are so small (less than
)
that they never interfere with the
identification of real frequencies.
Figure 2 shows the C OR OT light
curve for KPD 0629-0016. Low-amplitude multiperiodic oscillations
with periodicities around
1-2 h are clearly seen. This behaviour
is typical of the sdB
stars pulsating in mid-order g-modes
on these timescales. We note that the complex interference pattern
indicates that a significant number of modes are likely to be involved in
the brightness modulation.
![]() |
Figure 3:
Lomb-Scargle periodogram
(LSP) in the 90-400 |
Open with DEXTER |
The Lomb-Scargle periodogram (LSP; Scargle 1982)
of the light curve confirms the
multiperiodic nature of the star brightness modulation (see Fig. 3).
The region of interest, where signal is clearly detected covers
the range 90-400 Hz. Other frequency regions up to the Nyquist limit
(
15 600
Hz) are to be found consistent with noise.
We applied the usual prewhitening
and nonlinear least squares fitting techniques to extract the frequencies.
We used a dedicated software, F ELIX (Frequency Extraction for
LIghtcurve eXploitation) developed by one of us (S.C.), that greatly eases and
accelarates the application of this procedure, especially
for space-based data. The procedure was performed with neither ambiguity nor
difficulty, thanks to the very high quality of the C OR OT data.
We identified 17 frequencies, fn, emerging above the usual 4.0detection threshold. Table 1 provides
the attributes (frequency, period, amplitude, phase, and signal-to-noise
ratio) and their error estimates (
,
,
,
and
,
respectively) resulting from the nonlinear least squares
fit. We note that the phase is relative to the beginning of the run. We find that
the noise level in this frequency range is extremely low, at
(57 ppm). After prewhitening the 17 frequencies, a clear impression remained
that power is still present in that particular frequency bandpass.
This was particularly noticeable when comparing to regions where, clearly, no
signal exists. While more uncertain (i.e.,
statistically less significant), we identified 7 additional peaks,
un, listed in Table 1, with amplitudes
between 3.6 and 4.0
,
that could potentially be real.
![]() |
Figure 4:
Distribution of the 24 periods listed in Table 1
(
f1-f17: red segments; u1-u7: dotted red segments)
compared to the |
Open with DEXTER |
The reconstructed light curve based on all the harmonic oscillations
given in Table 1 is shown in Fig. 2
plotted (in red) over the observed light curve. In a similar way, Fig. 3 shows the Lomb-Scargle periodogram
of the observed time series in the 90-400 Hz window and,
plotted upside-down, its reconstruction based on all frequencies from
Table 1. The LSP of the residual light
curve after subtraction of all oscillations is also given, shifted downward.
In both the time domain (Fig. 2) and
frequency space (Fig. 3),
the reconstruction based on the fitted modulations closely reproduces the observations.
3 Comparison with a representative model
Table 1:
List of frequencies detected above
the local noise (fn) and above
the noise
(un). n is by order of decreasing amplitude.
Figure 4 shows the period distribution given in Table
1 compared to the low-degree (
;
the
most likely observed modes for visibility reasons) g-mode pulsation
spectrum of a model representative of KPD 0629-0016. We used our so-called
third generation models (Brassard & Fontaine 2008,2009), assuming a typical stellar
mass
,
a fractional mass of the outer hydrogen rich
envelope
,
a fractional mass of the convective
core
,
and a core C+O mass fraction
.
This structure leads to surface parameters
and
K that are close to the atmospheric values
estimated from spectroscopy for KPD 0629-0016. We emphasize that it does
not constitute a seismic solution of the observed period spectrum for this
star. An accurate determination of the star structural parameters is beyond
the scope of the present paper. This model simply provides a preliminary
insight into the expected mode structure.
Figure 4 indicates that the observed modes in KPD 0629-0016 are
relatively sparsely distributed compared to the computed spectrum.
Groups of modes with possibly consecutive radial orders, k, are visible (in
the
2500-4000 s range for instance), but between these groups some regions
show no detectable pulsations even though the theoretical spectrum has modes
in these area. The observed periods correspond to g-modes of radial order
k between 9 and 44, if ,
and between 16 and 76, if
.
The
mode density observed in the groups suggests that we are most likely observing a
combination of
and
nonradial oscillations. Although the
radial orders involved indicate that we are close to the asymptotic regime
(except, maybe, for the lowest period modes), any attempt to isolate typical
period spacings is problematic because sdB stars, in contrast to main
sequence stars, are stratified objects with at least two zones of
chemical transition in their interior producing significant trapping effects
on g-modes. These combined effects perturb the simple uniform
distribution predicted by the asymptotic pulsation theory as can be seen
in Fig. 4. We, indeed, find no clear spacings between
the periods given in Table 1. This is really not
surprising because, in addition, only a fraction of the expected
modes (according to the model) are effectively detected. The only reliable
approach will be to match the pulsation spectrum in a detailed asteroseismic analysis.
4 Conclusion and prospects
The
More generally, CoRoT demonstrates that contemporary high accuracy photometry
obtained in space provides a decisive breakthough in g-mode sdB pulsator
characterisation. The key combined factors are the improved
sensitivity, leading in the present case to a noise level of only 57 ppm in
Fourier space, and the very high duty cycle that eliminates the aliasing
problem. Even if large campaigns
such as those of Randall et al. (2006b,a) and
Baran et al. (2009) detected fairly large numbers of modes for some
of the brightest known pulsators, the determination of their
frequencies is unreliable for a large part, due to diurnal aliasing.
This has catastrophic consequences for detailed asteroseismic studies because
one daily alias (
Hz) mistakenly identified as the true
frequency introduces an error between 3 and 12% for modes in the 400-100
Hz range. No reliable asteroseismic solution can be secured under
these conditions (we recall that current asteroseismic solutions for
sdB stars reach a typical accuracy of
).
With the present CoRoT observations of KPD 0629-0016, we have at last obtained a
number of detected modes with secured frequencies, which, from our
experience, is sufficient to provide a very solid basis for a detailed
asteroseismic study similar to those conducted for the short period p-mode
pulsators.
CoRoT is undoubtedly opening a new era in hot B subdwarf asteroseismology
that will permit the exploitation of g-modes as potential deep probes of the
internal structure of these stars. We have embarked in this venture by targeting KPD
0629-0016 and other stars of similar type that will benefit from space
observations with the C OR OT and Kepler satellites.
References
- Auvergne, M., Bodin, P., Boisnard, L., et al. 2009, A&A, 506, 411 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Baglin, A., Auvergne, M., Barge, P., et al. 2006, ESA Spec. Publ., 1306, 33 [Google Scholar]
- Baran, A., Oreiro, R., Pigulski, A., et al. 2009, MNRAS, 392, 1092 [NASA ADS] [CrossRef] [Google Scholar]
- Brassard, P., & Fontaine, G. 2008, ASPC, 392, 261 [Google Scholar]
- Brassard, P., & Fontaine, G. 2009, JPhCS, 172, 2016 [NASA ADS] [Google Scholar]
- Charpinet, S., Brassard, P., Fontaine, G., et al. 2009, in AIPCS 1170, ed. J. A. Guzik, & P. A. Bradley, 585 [Google Scholar]
- Charpinet, S., Fontaine, G., Brassard, P., & Dorman, B. 1996, ApJ, 471, L103 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
- Charpinet, S., Fontaine, G., Brassard, P., et al. 1997, ApJ, 483, L123 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
- Charpinet, S., Fontaine, G., Brassard, P., & Dorman, B. 2000, ApJS, 131, 223 [Google Scholar]
- Charpinet, S., van Grootel, V., Reese, D., et al. 2008, A&A, 489, 377 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- D'Cruz, N. L., Dorman, B., Rood, R. T., & O'Connell, R. W. 1996, ApJ, 466, 359 [NASA ADS] [CrossRef] [Google Scholar]
- Fontaine, G., Brassard, P., Charpinet, S., et al. 2003, ApJ, 597, 518 [Google Scholar]
- Gilliland, R. L., Brown, T. M., Christensen-Dalsgaard, J., et al. 2010, PASP, 122, 131 [NASA ADS] [CrossRef] [Google Scholar]
- Green, E. M., Fontaine, G., Reed, M. D., et al. 2003, ApJ, 583, L31 [NASA ADS] [CrossRef] [Google Scholar]
- Han, Z., Podsiadlowski, P., Maxted, P. F. L., et al. 2002, MNRAS, 336, 449 [NASA ADS] [CrossRef] [Google Scholar]
- Han, Z., Podsiadlowski, Ph., Maxted, P. F. L., & Marsh, T. R. 2003, MNRAS, 341, 669 [NASA ADS] [CrossRef] [Google Scholar]
- Kilkenny, D., Koen, C., O'Donoghue, D., & Stobie, R. S. 1997, MNRAS, 285, 640 [Google Scholar]
- Koen, C., & Green, E. M. 2007, MNRAS, 377, 1605 [NASA ADS] [CrossRef] [Google Scholar]
- Randall, S. K., Matthews, J. M., Fontaine, G., et al. 2005, ApJ, 633, 460 [NASA ADS] [CrossRef] [Google Scholar]
- Randall, S. K., Fontaine, G., Green, E. M., et al. 2006a, ApJ, 643, 1198 [NASA ADS] [CrossRef] [Google Scholar]
- Randall, S. K., Green, E. M., Fontaine, G., et al. 2006b, ApJ, 645, 1464 [NASA ADS] [CrossRef] [Google Scholar]
- Randall, S. K., van Grootel, V., Fontaine, G., Charpinet, S., & Brassard, P. 2009, A&A, 507, 911 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Scargle, J. D. 1982, ApJ, 263, 835 [NASA ADS] [CrossRef] [Google Scholar]
- Schuh, S., Huber, J., Dreizler, S., et al. 2006, A&A, 445, L31 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- van Grootel, V., Charpinet, S., Fontaine, G., & Brassard, P. 2008a, A&A, 483, 875 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- van Grootel, V., Charpinet, S., Fontaine, G., et al. 2008b, A&A, 488, 685 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
Online Material
Appendix A: Details on the CoRoT time series analysis and frequency extraction
We provide additional technical details about the data acquisition, light curve analysis, and frequency extraction.
KPD 0629-0016 was monitored with a sampling time of 32 s. At each measurement, a small image (also called ``imagette'') of the star was acquired, kept on board and transferred to the operation center. In the planet finding channel, the instrument is equipped with a small prism that produces a very low resolution spectrum on the CCD. When building photometric light curves, the pixel counts in the ``imagette'' are integrated into three separate time series corresponding to different colors (``red'', ``green'', and ``blue''). It is similar to broadband photometry, although with a limited wavelength coverage. Future exploitation of this color information needs to be investigated, but since our primary goal was to decrease as much as possible the noise level, we focused on the white light photometry produced by adding the counts of each color light curve at each time step.
For the analysis, we kept only data points that did not suffer from
any reported perturbation (i.e., with flag set to zero). This excludes,
most notably, measurements obtained during cyclic satellite transits
across the South Atlantic Anomaly, which considerably degrades the
accuracy and produces significant pollution in the data. The light
curve was further de-trended for residual long term variations. Data points
that significantly differ from the local standard deviation
were removed by applying a running 3-
clipping filter.
After all this filtering, the effective duty cycle dropped from
99.3% for the original data to
80.5%, but the periodograms are
then vastly improved. The price to pay is the presence of side lobes
in the window function, which, however, remain small and do not disturb
the identification of true frequencies.
We applied the usual prewhitening technique to extract frequencies, one at time: at each step of the procedure, the Lomb-Scargle periodogram (LSP) is computed, the highest peak in the frequency domain is selected and a cosine wave is fitted for frequency, amplitude, and phase to the time series data using a Levenberg-Marquardt nonlinear least squares algorithm. The fitted cosine wave is then subtracted from the data, resulting in the removal of the corresponding peak in the LSP (as well as its associated side lobes due to the window). The procedure is repeated until no more signal emerges over a given detection threshold above the noise (see below). Repeatedly, and at the end of the procedure, all cosine waves are fitted simultaneously using the nonlinear least squares routine.
![]() |
Figure A.1: Lomb-Scargle periodograms (showing amplitude spectra in % of the mean brightness) and residuals after prewhitening of the identified frequencies in the CoRoT time series. The green (blue, red) dotted curves refer to a value equal to 4.0 (3.6, 3.0) times the local median noise level. Vertical dotted lines indicate the position of a peak previously removed. |
Open with DEXTER |
The prewhitening sequence is illustrated in Fig. A.1.
The top panel shows the LSP of the light curve in the 90-400 Hz
range. The 5 dominant (in amplitude) frequencies are indicated and
the result of their prewhitening is then plotted in the panel below.
The procedure continues with 4 additional frequencies removed, leading
to the LSP represented in the panel further down. The frequency selection
and removal proceeds until peaks no longer emerge above the detection
threshold. Various thresholds are shown in Fig. A.1
corresponding to 4 times (green), 3.6 times (blue), and 3 times (red)
the median noise level, respectively. This noise level is estimated
locally by computing the median amplitude value within a running box
of width
1000 frequency bins in the LSP. It is re-evaluated
after each frequency prewhitening, such that, at the end of the procedure,
the value obtained is representative of the true local noise contribution
separated from the signal window pattern, even in crowded frequency
regions containing several close peaks.
It is usual to adopt the ``4-
criterion'' as a safe limit
for the detection of frequencies, which roughly corresponds to the
99.9% statistical significance level. All the peaks indicated in
the 3 successive panels from the top in Fig. A.1 conform
to this criterion, leading to the detection of 17 frequencies, fn,
given in Table 1. The 7 additional peaks, un,
are identified in the fourth panel (from the top) in
Fig. A.1. These have amplitudes between 3.6 and 4.0
.
Footnotes
- ... asteroseismology
- The CoRoT space mission, launched on December 27th 2006, has been developed and is operated by CNES, with the contribution of Austria, Belgium, Brazil, ESA, Germany, and Spain.
- ...
- Appendix A is only available in electronic form at http://www.aanda.org
All Tables
Table 1:
List of frequencies detected above
the local noise (fn) and above
the noise
(un). n is by order of decreasing amplitude.
All Figures
![]() |
Figure 1: Location of KPD 0629-0016 (black open circle) in the effective temperature-surface gravity plane in relation to the other known pulsating hot B subdwarf stars using a sample with reliable and homogeneous spectroscopic measurements. The short-period p-mode pulsators (V361 Hya stars) are shown in blue, while the long-period g-mode pulsators (V1093 Her stars) are shown in red. The three known hybrid stars showing simultaneously both p-mode and g-mode oscillations are also clearly indicated at the commun boundary of both classes. KPD 0629-0016 belongs to the V1093 Her family. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: C OR OT light curve obtained for KPD 0629-0016 during SRa03 observations. The amplitude as a function of time is expressed in terms of the residual intensity relative to the mean brightness of the star. The red curve in the bottom panel shows the reconstructed signal based on the extracted frequencies, amplitudes, and phases given in Table 1. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Lomb-Scargle periodogram
(LSP) in the 90-400 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Distribution of the 24 periods listed in Table 1
(
f1-f17: red segments; u1-u7: dotted red segments)
compared to the |
Open with DEXTER | |
In the text |
![]() |
Figure A.1: Lomb-Scargle periodograms (showing amplitude spectra in % of the mean brightness) and residuals after prewhitening of the identified frequencies in the CoRoT time series. The green (blue, red) dotted curves refer to a value equal to 4.0 (3.6, 3.0) times the local median noise level. Vertical dotted lines indicate the position of a peak previously removed. |
Open with DEXTER | |
In the text |
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