A. Aungwerojwit1 - B. T. Gänsicke1 - P. Rodríguez-Gil1,2 - H.-J. Hagen3 - E. T. Harlaftis4 - C. Papadimitriou5,6 - H. Lehto7,8 - S. Araujo-Betancor9 - U. Heber10 - R. E. Fried11 - D. Engels3 - S. Katajainen8
1 -
Department of Physics, University of Warwick, Coventry CV4 7AL, UK
2 -
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
3 -
Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg
112, 21029 Hamburg, Germany
4 -
Institute of Space Applications and Remote Sensing,
National Observatory of Athens, PO Box 20048, Athens 11810, Greece
5 -
Institute of Astronomy and Astrophysics,
National Observatory of Athens, PO Box 20048, Athens 11810, Greece
6 -
Department of Astrophysics, Astronomy and Mechanics, University of Athens,
157 84 Zografos, Athens, Greece
7 -
Department of Physics, 20014 University of Turku, Finland
8 -
Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
9 -
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD21218, USA
10 -
Astronomisches Institut der Universität Erlangen,
Germany
11 -
Braeside Observatory, PO Box906, Flagstaff, AZ 86002, USA
Received 23 December 2004 / Accepted 3 July 2005
Abstract
We present time-resolved optical spectroscopy and
photometry of four relatively bright (
)
long-period
cataclysmic variables (CVs) discovered in the Hamburg Quasar Survey:
HS 0139+0559, HS 0229+8016, HS 0506+7725, and HS 0642+5049. Their
respective orbital periods,
min,
min,
min, and
min are determined from
radial velocity and photometric variability studies. HS 0506+7725
is characterised by strong Balmer and He emission lines,
short-period (
10-20 min) flickering, and weak X-ray emission
in the ROSAT All Sky Survey. The detection of a deep low state
(
)
identifies HS 0506+7725 as a member of the VY Scl
stars. HS 0139+0559, HS 0229+8016, and HS 0642+5049 display
thick-disc like spectra and no or only weak flickering activity.
HS 0139+0559 and HS 0229+8016 exhibit clean quasi-sinusoidal
radial velocity variations of their emission lines but no or very
little orbital photometric variability. In contrast, we detect no
radial velocity variation in HS 0642+5049 but a noticeable orbital
brightness variation. We identify all three systems either as
UX UMa-type novalike variables or as Z Cam-type dwarf novae. Our
identification of these four new systems underlines that the
currently known sample of CVs is rather incomplete
even for bright objects. The four new systems add to the clustering of
orbital periods in the 3-4 h range found in the sample of HQS
selected CVs, and we discuss the large incidence of magnetic CVs and
VY Scl/SW Sex stars found in this period range among the known
population of CVs.
Key words: stars: binaries: close - stars: individual: HS 0139+0559 - stars: individual: HS 0229+8016 - stars: dwarf novae - stars: individual: HS 0506+7725 - stars: individual: HS 0642+5049 - stars: novae, cataclysmic variables
About 75% of all known CVs have been discovered either because of their variability or because of their X-ray emission, with a strong dominance of dwarf novae and classical novae in the first group and magnetic CVs in the second group (Gänsicke 2005). It is therefore clear that CVs characterised by infrequent outbursts and/or low-amplitude variability, as well as lacking strong X-ray emission, will be underrepresented in the currently known CV population. Hypothetically, such objects could make up for the large number of predicted short-period CVs with low mass transfer rates.
We are currently carrying out a spectroscopic survey of CV candidates with the aim to produce a homogeneously selected sample of CVs that overcomes the observational biases mentioned above. In this paper, we describe the status of our survey, the identification of four new CVs with periods close to 4 h, and discuss the nature of CVs in the 3-4 h period range.
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Figure 1:
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Our CV candidate selection made use of a property that has never been
systematically exploited before: the spectroscopic hallmark
of CVs, i.e. the presence of noticeable emission lines in most
CVs. The CV candidate selection was carried out by visually inspecting
48 708 HQS prism spectra for the presence of Balmer emission
lines. In order to test the efficiency of this method, we applied the
same procedure to the subset of 84 previously known CVs
(Downes et al. 2001, as of July 2001) that are contained in the
HQS spectral data base. We positively recovered 90% of the
known short-period (
h) CVs, including prominent dwarf novae
such as SW UMa or T Leo (the latter one has a
rather long outburst cycle of
420 d), as well as magnetic CVs,
such as AN UMa or ST LMi. The fraction of recovered systems drops to
40% for long-period systems, with the largest fraction of
missed identifications being novalike variables with weak or no Balmer
emission lines. In total, 62% of the previously known CVs with an
HQS prism spectrum were positively identified by our selection
method. We concluded from this test that the HQS should be
very efficient in finding CVs below the period gap, as long as they have
similar spectroscopic properties to the previously known systems, i.e.
Balmer emission lines with equivalent widths >10 Å in
.
The
decrease in detection efficiency for long-period systems has been
compensated to some extent by follow-up programs investigating hot stars in the
HQS, delivering a number of new CVs with weak emission lines
(Heber et al. 1991).
In total, 53 new CVs were identified within this project, and substantial observational effort was invested in determining the properties of these systems. To date, 42 HQS CVs have had their orbital period measured. Despite its good sensitivity for short-period CVs, only a small number of new short-period CVs have been found, and those that have been found fully confirm the expected properties: low-amplitude variability and/or long outburst recurrence times, e.g. KV Dra (HS 1449+6415, Nogami et al. 2000), HS 2331+3905 (Araujo-Betancor et al. 2005a), DW Cnc (HS 0756+1624, Rodríguez-Gil et al. 2004a), or HS 2219+1824 (Rodríguez-Gil et al. 2005a). The majority of the newly identified systems have orbital periods above the 2-3 h period gap, including rarely outbursting dwarf novae such as GY Cnc (HS 0907+1902, Gänsicke et al. 2000) or RX J0944.5+0357 (HS0941+0411, Mennickent et al. 2002); magnetic CVs with relatively weak X-ray emission, such as 1RXS J062518.2+733433 (HS0618+7336, Araujo-Betancor et al. 2003), RX J1554.2+2721 (HS 1552+2730, Thorstensen & Fenton 2002; Gänsicke et al. 2004b), and HS 0943+1404 (Rodríguez-Gil et al. 2005b); and half a dozen new SW Sextantis stars (Szkody et al. 2001; Rodríguez-Gil 2005; Gänsicke et al. 2002c; Rodríguez-Gil et al. 2004b).
While a full discussion of the implications that our search for CVs in
the HQS has for our understanding of the galactic CV population has to
await characterisation of the full HQS CV sample, an important
preliminary statement that will not substantially change is:
there is no large population of nearby short-period CVs
that resemble the known template systems. Phrased differently, if the
large population of short period CVs predicted by theory exists, the
majority of these systems must look different from the known
short-period systems, i.e. have weak emission lines and/or
substantially redder continua. Interestingly, the Sloan Digital Sky
Survey (SDSS) is discovering a number of CVs with a steep Balmer
decrement, in which the white dwarf dominates the optical emission, a
clear sign of low mass transfer rates (Szkody et al. 2005, and references
therein). However, most of these systems are very
faint,
,
implying that they are distant (d>100 pc)
and not intrinsically numerous anywhere near the numbers predicted by
theory.
An unexpected finding of our search for CVs in the HQS has been the identification of a large number of systems with orbital periods in the range 3-4 h. Here, we report the discovery of four additional CVs in that period range, HS 0139+0559, HS 0229+8016, HS 0506+7725, and HS 0642+5049. Despite being close in orbital period, these systems differ dramatically in their observed characteristic, and we discuss in detail the properties of the known CVs in the 3-4 h period range.
HS 0229+8016 (Fig. 1) was first spectroscopically observed
at the Calar Alto 2.2 m telescope on August 8, 1992, using the Boller
& Chivens spectrograph. The spectrum (Fig. 3,
top panel) shows a blue continuum with the Balmer jump in emission,
superimposed by moderately strong Balmer emission lines. The higher
Balmer line profiles show evidence of a P-Cygni like structure with
blue absorption wings increasing in strength for the higher members of
the series. He I 4471 is observed in absorption, and an
emission line near 4630 Å is detected, which we tentatively
identify as the N/C Bowen blend emission. Rather unusual is, however,
the fact that He II
4686 is not detected in emission along
with the Bowen blend. HS 0229+8016 was observed again at the Calar
Alto 2.2 m telescope on October 5, 1998, using the Calar Alto Faint
Object Spectrograph (CAFOS), on this occasion looking nearly identical
to HS 0139+8016 (Fig. 3, bottom panel). The
spectral characteristics and the variability clearly identify
HS 0229+8016 as a CV.
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Figure 2: Identification spectrum of HS 0139+0559 obtained at the Calar Alto 3.5 m telescope on January 22, 1989. |
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Figure 3: Identification spectra of HS 0229+8016 obtained at the Calar Alto 2.2 m telescope on August 8, 1992 ( top panel) and February 2, 1998 ( bottom panel). |
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Figure 4: Identification spectrum of HS 0506+7725 obtained at the Calar Alto 2.2 m telescope on February 2, 1998. |
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An identification spectrum of HS 0506+7725 (Fig. 1) was
obtained on February 2, 1998, on the Calar Alto 2.2 m telescope using
CAFOS with the B-400 grism. The spectrum (Fig. 4)
displays a blue continuum with the Balmer and Paschen jumps in
emission, plus emission lines of hydrogen and He I. The
N/C Bowen blend and the He II
4686
emission lines are also detected. The strength of these
high-ionisation lines is typical of novalike variables, magnetic CVs,
or nova remnants.
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Figure 5: Average of the 17 CAFOS G-100 spectra of HS 0642+5049 obtained at the Calar Alto 2.2 m telescope on October 24, 2004. |
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HS 0642+5049 (Fig. 1) was spectroscopically identified as a
CV on March 7, 1999, with CAFOS at the Calar Alto 2.2 m telescope. The
spectrum of HS 0642+5049 (Fig. 5) contains
a blue continuum with moderately strong
emission. The
and
emission is embedded in broad absorption troughs, and weak
He I
4471 absorption is also detected. No He II
4686
and
N/C Bowen blend are detected.
In order to determine the orbital periods of HS 0139+0559,
HS 0229+8016, HS 0506+7725, and HS 0642+5049, we obtained
time-resolved spectroscopy at Calar Alto Observatory and Roche de los
Muchachos Observatory throughout the period October 2002 to October
2004 (Table 1). At the Calar Alto 2.2 m telescope, we
used the CAFOS spectrograph equipped with the G-100 grating and a
pixel SITe CCD. This setup, in conjunction
with a 1.2
slit, provided a spectral resolution of
4.1 Å (full width at half maximum, FWHM) covering the range
4240-8300 Å. We used the double-armed TWIN spectrograph at the
Calar Alto 3.5 m telescope equipped with the T05 grating in the blue
and the T06 grating in the red, providing a spectral resolution of
1.2 Å (FWHM) in the ranges
and
.
At the 2.5 m Isaac Newton Telescope (INT),
we used the Intermediate Dispersion Spectrograph (IDS) equipped with
the R632V grating and the
pixel EEV10a
detector. Using a slit width of 1.5
this setup provided a
useful wavelength of
4400-6800 Å and a spectral
resolution of
2.3 Å. Arc calibration spectra were interleaved
with the target observations every
40 min.
In total, we obtained 55 spectra of HS 0139+0559, 74 spectra of
HS 0229+8016, 86 spectra of HS 0506+7725, and 87 spectra of
HS 0642+5049 (Table 1). Reduction of the follow-up
spectroscopy consisting of bias and flat-field corrections and optimal
extraction (Horne 1986) was carried out in
IRAF. The wavelength calibration of the
extracted spectra was performed in MOLLY. The dispersion
relation was obtained by fitting a low-order polynomial to the arc
lines, with the rms less than one tenth of the dispersion in all
cases. The flexure of the telescope was accounted for by interpolating
between the two arc exposures bracketing the target spectra.
The time-resolved spectra of HS 0139+0559 and HS 0229+8016 are similar
to the identification spectra shown in Figs. 2
and 3 (bottom panel), with
and
in
absorption for most of the time and occasionally showing signs of
emission cores. However, the G-100 spectra cover
as well, which
is observed in emission throughout, with low equivalent widths in the
range 3-6 Å. The time-resolved spectra of HS 0506+7725 are very
similar to the identification spectrum shown in
Fig. 4, with the difference of having no
He II
4686 emission. The
emission line has an average
equivalent width of
15 Å. The time-resolved spectra of
HS 0642+5049 show
in emission with an average equivalent width
of
5 Å, and weak
and
emission embedded in broad
absorption lines (Fig. 5).
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Figure 6: Differential CCD R-band ( top panel) and B-band ( bottom panel) photometry of HS 0139+0559 obtained at the Braeside observatory. |
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Figure 7: Sample light curves of HS 0229+8016. Top panel: R-band data obtained at the Kryoneri observatory. Bottom panel: filterless data obtained at the Tuorla observatory. |
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Figure 8: Sample light curves of HS 0506+7725. Top panel: R-band data obtained at the Kryoneri observatory. Bottom panel: filterless data obtained at the OGS telescope. |
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Figure 9: Sample filterless light curves of HS 0642+5049 obtained at the IAC80 telescope. |
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The USNO-A2.0 catalogue lists HS 0139+0559 with B=R=14.4, and we
found
and
in our
observations. HS 0229+8016 has B=13.9 and R=13.8 in the USNO-A2.0
catalogue, and it was found during our observations mostly near
,
except on one occasion (August 1992) when
it was as faint as
mag. HS 0506+7725 is listed with
B=15.3 and R=15.6 in the USNO-A2.0 catalogue, and our data
provides evidence of long-term variability of the mean magnitude by
1 mag, but it dropped on one occasion (October 1995) into a deep
low state at
.
HS 0642+5049 is found in the USNO-A2.0
catalogue with B=16.6 and R=16.9, and we found
mag without significant long-term variability of
the system.
Table 2:
Sine fits to the
emission line radial
velocities. The methods employed were a convolution
with a single Gaussian (SG) or Schneider & Young's
(1980) double-Gaussian prescription (DG).
For completeness, we also used the Braeside B and R band photometry for a period analysis. As suggested by the flat light curve (Fig. 6), no significant signal is detected in the Scargle periodogram.
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Figure 10:
The Scargle periodogram of the
radial velocities of HS 0139+0559 measured from the
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Figure 11:
The Scargle periodogram of the
radial velocities of HS 0229+8016 measured from the
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Scargle periodograms computed from the two longest photometry runs on
HS 0229+0559 are dominated by a broad signal near 5.2
(Kryoneri
data) and 6.2
(Tuorla), which are consistent with the
spectroscopic period or its one-day alias. While our photometric data
is not sufficient to improve the period determination of
HS 0229+0559, it suggests that the orbital period of HS 0229+0559
can be refined by a sequence of sufficiently long photometric
time-series.
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Figure 12:
The Scargle periodogram of the
radial velocities of HS 0506+7725 measured from the
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Our time-series analysis of the photometry of HS 0506+7725 did not lead to the detection of any significant signal, either at long (orbital) or at short (putative white dwarf spin) frequencies, making the observed short-term variability (Fig. 8) a nice example of non-coherent CV flickering.
Considering the 3.5 h modulation observed in the HS 0642+5049
light curves, we used the three longest and closest spaced photometric
data sets obtained at the Calar Alto 2.2 m telescope (October
25, 2004) and the IAC80 (December 8 and 9, 2004) to determine the orbital
period of the system. The strongest peak detected in the Scargle
periodogram computed from these data is found at
,
surrounded by one-day aliases
(Fig. 13). In order to test the significance
of the signal, we created a set of fake data from a sine wave with a
frequency of 6.3746 evaluated at the exact times of the
observations. The alias structures of the periodograms calculated from
the fake data and the real data agree well. We conclude that the
orbital period of HS 0642+5049 is
min.
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Figure 13: The Scargle periodogram of HS 0642+5049 computed from the three longest nights of differential photometry obtained at the Calar Alto (October 25, 2004) and the IAC80 (December 8 and 9, 2004). The periodogram from a fake data set assuming a period of 225.90 min is shown in the top panel. |
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Figure 14:
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Figure 15:
HS 0642+5049 photometric data from the
CAFOS (October 9, 2004) and the IAC80 (December 8 and 9, 2004) folded on
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Figure 15 shows the CAFOS and the IAC80 photometry folded on 225.90 min and averaged into 30 phase bins, which reflects the morphology of the individual light curves (Fig. 9).
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Figure 16: The period distribution of 42 new CVs discovered in the HQS ( left panel) and of all known CVs ( right panel, from Ritter & Kolb 2003, V7.4). The 2-3 h period gap is shaded in gray. |
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Table 3: Comparison of the observational characteristics of the four new CVs. The (non)detection of X-ray emission refers to the ROSAT All Sky Survey (Voges et al. 2000). The CV subtypes are abbreviated as UX = UX UMa type novalike variable, ZC = Z Cam type dwarf nova, VY = VY Scl star.
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Figure 17: The period distribution of the individual CV subtypes in the 3-4 h period range. From top to bottom: polars (am), intermediate polars (ip), VY Scl and SW Sex stars (vy), novalike variables and nova remnants that are neither VY Scl nor SW Sex stars (nl), U Gem type dwarf novae (ug), Z Cam type dwarf novae (zc), SU UMa type dwarf novae (su), and systems with undetermined CV subtype (xx). |
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The HQS CV survey has been very prolific in identifying relatively
bright long-period CVs with a distinct preference for the 3-4 h
period range (Fig. 16), including the four new CVs
presented in this paper. The majority of these new long-period CVs are
weak or no X-ray emitters, and they display little long-term
variability - in fact, only five confirmed dwarf novae are among the
28 new systems found above the gap. Gänsicke (2004) and
Rodríguez-Gil (2005) pointed out the large
number of SW Sextantis stars among the new HQS CVs, which
represent 25% of all newly identified CVs above the gap, and nearly
half of all the new CVs in the 3-4 h period range. For comparison,
we show in Fig. 17 the inventory of the 3-4 h orbital
period range according to Ritter & Kolb
(2003, V7.4). We find that 114 CVs (20% of all
CVs with known
)
inhabit the 3-4 h period range, of which 27 (24%)
are confirmed magnetic systems (intermediate polars,
polars). 33 (29%) belong to the group of either VY Scl or SW Sex
stars, which share similar properties, and are suspected to contain
magnetic white dwarfs as well (e.g. Rodríguez-Gil et al. 2001; Hameury & Lasota 2002). While the ratio of definite magnetic CVs in the
3-4 h period range (24%) is already very high compared to the
incidence of magnetism in isolated white dwarfs
(Liebert et al. 2003), confirmation of significant magnetism in the
white dwarfs in VY Scl/SW Sex stars would raise the ratio of
magnetic/non-magnetic CVs well above 50%, which conflicts with any of
the current models of CV evolution. Whatever the verdict on the
magnetic fields in VY Scl/SW Sex stars will be, the large number of CVs
belonging to this type suggests that they represent an important phase
of CV evolution rather than some unusual combination of their physical
properties. For completeness, 32 (28%) novalike variables that do not
belong to either the VY Scl or SW Sex class populate the 3-4 h
period range
. While a number of those systems definitely
do not share any of the VY Scl/SW Sex properties, a fair fraction of
these systems has been studied only in a very limited way, and hence
some of them might well join the VY Scl/SW Sex class upon more detailed
scrutiny. Finally, 16 (14%) dwarf novae are known in the 3-4 h
period range, and the scarcity of systems undergoing thermal disc
instabilities just above the period gap is a well-known fact
(e.g. Shafter 1992; Shafter et al. 1986).
HS 0506+7725 shows short time scale flickering with quasi-periodic
oscillations on time scales of 15 min. The relatively narrow
emission lines and the low amplitude of the radial velocity variations
suggest a low inclination. The system has been detected in the RASS
(Voges et al. 2000) at
(1RXS J051336.1+772836) with a hard spectrum, and has been
previously detected as an X-ray source by EINSTEIN
(2E 0506.1+7725). The presence of moderately strong
He II
4686 emission in the identification spectrum
independently confirms the presence of ionising radiation in the
system. The detection of a deep low state at
on one of
the HQS prism plates clearly identifies the system as a VY Scl
star. The system does not display evidence at face value of
being an SW Sex star; but as it is obviously a low-inclination binary, a
spectroscopic study at higher resolution would be useful to test for
anomalous radial velocity behaviour in the emission lines.
The other three systems, HS 0139+0559, HS 0229+8016, and
HS 0642+5049 are spectroscopically very similar, being characterised
by thick-disc absorption line spectra. The fact that we have observed
them on various occasions and found them always at nearly the same
magnitude and with the same spectral properties makes it very unlikely that
these systems are U Gem-type dwarf novae observed during
outburst. While HS 0139+0559 and HS 0229+8016 are not detected in
the RASS, a faint X-ray source is found near HS 0642+5049
(1RXS J066618.4+504601,
), which
coincides within the 29
position error of the RASS detection
with the CV. The fact that there are no other nearby objects suggests
that HS 0642+5049 is a weak X-ray emitter. None of the systems shows
strong flickering activity. One puzzling difference among the three
systems is that, whereas HS 0139+0559 and HS 0229+8016 show no or
only very low-amplitude orbital photometric variability but exhibit
clean quasi-sinusoidal radial velocity variations in their emission
lines, HS 0642+5049 does not display any radial velocity variation
but shows a 0.2 mag photometric modulation. It is very
difficult to reconcile this opposite difference in
spectroscopic/photometric behaviour in the simple picture of a
high-mass transfer CV with a steady-state accretion disc. Based on our
data, we identify all three systems either as UX UMa-type novalike
variables or as Z Cam-type dwarf novae observed in periods of
standstill. Optical long-term monitoring will be necessary to
distinguish between these two possibilities.
Acknowledgements
A.A. thanks the Royal Thai Government for a studentship. B.T.G. and P.R.G. were supported by a PPARC Advanced Fellowship and a PDRA grant, respectively. The HQS was supported by the Deutsche Forschungsgemeinschaft through grants Re 353/11 and Re 353/22. Tom Marsh is acknowledged for developing and sharing his reduction and analysis package MOLLY. We thank the referee, Mike Shara, for his comments that lead to an improved presentation of the paper.
Table 1: Log of the observations.