# The Radio Continuum Source Projected Near HR 8799

L. F. Rodríguez11affiliation: Instituto de Radioastronomía y Astrofísica,
UNAM, México. 22affiliation: Mesoamerican Center for Theoretical Physics,
UNACH, México L. A. Zapata11affiliationmark:  Luis F. Rodríguez and Luis A.
Zapata: Instituto de Radioastronomía y Astrofísica, Universidad Nacional
Autónoma de México, Apartado Postal 3–72, 58090 Morelia, Michoacán, México
(l.rodriguez@irya.unam.mx).

###### Abstract

HR 8799 is an A5/F0 V star where exoplanets were first directly imaged. Four
exoplanets were found within $\simeq 2\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}0$ from the star. Here we report the VLA
detection of a faint (19.1$\pm$2.7 $\mu$Jy) radio continuum (3.0 GHz) source
projected at $\simeq 2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ from the star.
The a priori probability of finding a background source with this flux density
within a radius of $2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ is only 0.0046.
However, the astrometry made with the VLA and ALMA images, separated by 5.5
years, indicates no significant proper motions and rules out the association
of the radio source with the HR 8799 system and suggests it is a background
millimeter galaxy with dust emission in the millimeter and partially thick
synchrotron emission in the centimeter.

HR 8799 es una estrella A5/F0 V donde se detectaron por primera vez
exoplanetas con la técnica de imágenes directas. Se encontraron cuatro
exoplanetas dentro de $\simeq 2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}0$ de la
estrella. Aquí reportamos la detección con el VLA de una fuente débil de radio
continuo (19.1$\pm$2.7 $\mu$Jy) a una frecuencia de 3.0 GHz proyectada a
$\simeq 2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ de la estrella. La
probabilidad a priori de encontrar una fuente de fondo con esta densidad de
flujo dentro de un radio de $2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ es
solo 0.0046. Sin embargo, la astrometría realizada con las imágenes del VLA y
ALMA, separadas por 5.5 años, indica que no hay movimientos propios
significativos y descarta la asociación de la fuente de radio con el sistema
HR 8799. y sugiere que se trata de una galaxia milimétrica de fondo con
emisión de polvo en el milimétrico y emisión sincrotrón parcialmente
ópticamente gruesa en el centimétrico.

Astrometry proper motions Radio continuum: general stars: individual (HR 8799)

##  0.1 Introduction

In excess of 100 exoplanets have been discovered with the technique of direct
imaging, as listed in the NASA Exoplanet Archive (Akeson et al. 2013). HR 8799
is the star where exoplanets were first directly imaged. Four exoplanets were
found within $\sim$80 au ($\sim 2^{\prime\prime}$ at the distance of 40.85 pc)
in the plane of the sky from the star (Marois et al. 2008; 2010). The observed
proper motions of the exoplanets around the star confirmed the association
(Close & Males 2010). In addition to the four exoplanets, HR 8799 exhibits a
debris disk (Booth et al. 2016; Wilner et al. 2018), extending from $\simeq
2^{\prime\prime}$ ($\simeq$80 au) to $\simeq 7^{\prime\prime}$ ($\simeq$280
au) in radius, with a clear central cavity and detectable in the sub-
millimeter (Faramaz et al. 2021). In addition, these authors report the
presence of an 880 $\mu$m point source associated with the star.

In this paper we present sensitive VLA observations of the HR 8799 region
obtained with the purpose of searching for emission from the star or from one
of its four exoplanets. In section 2 we discuss the observations, while in
section 3 we interpret the data. Finally, our conclusions are presented in
section 4.

##  0.2 Observations

###  0.2.1 VLA

The data of project 12B-188 was obtained from the archives of the Karl G.
Jansky Very Large Array (VLA) of NRAO111The National Radio Astronomy
Observatory is a facility of the National Science Foundation operated under
cooperative agreement by Associated Universities, Inc.. These observations
were made in the highest angular resolution A configuration during six epochs
between 2012 October 31 and 2012 November 26. The observations were made in
the S-band continuum (2-4 GHz), with 16 spectral windows of 128 MHz each.
These spectral windows were divided in 64 channels of 2 MHz individual width.
The amplitude calibrator was J0137+3309 (3C48) and the gain calibrator was
J2254+2445. The data were calibrated in the standard manner using the version
5.6.2-3 of the CASA (Common Astronomy Software Applications; McMullin et al.
2007) package of NRAO and the pipeline provided for
VLA222https://science.nrao.edu/facilities/vla/data-processing/pipeline
observations. The data of the six epochs were concatenated in a single file to
increase the signal-to-noise ratio. The images were made using a robust
weighting of 2 (Briggs 1995), to optimize the sensitivity at the expense of
losing some angular resolution.

###  0.2.2 ALMA

The sub-millimeter continuum data (at 340 GHz) were obtained from the ALMA
data archive. The project was made under the program 2016.1.00907.S (PI: V.
Faramaz). For this study, we only used the 12 m array observations carried out
from 2018 May 13 to June 1. The data were taken using baselines ranging from
15 to 314 m (18 to 392 k$\lambda$). The ALMA digital correlator was configured
with four spectral windows (SPWs), each one 2 GHz wide. Three of these SPWs
were used for the continuum, and one for the detection of the CO(3$-$2)
molecular line at a rest frequency of 345.79598990 GHz. Bright quasars
J2148$+$0657, J2253$+$1608, and J2253$+$1608 were used as flux, bandpass, and
gain phase calibrators. The total time on-source was 4.5 hrs. The raw data
were calibrated, and then imaged using the Common Astronomy Software
Applications (CASA) version 5.1.1. The digital correlator was set up with
three spectral windows with a bandwidth of 2 GHz (divided by 128 channels
resulting in a channel width of 15.625 MHz) and one spectral window with a
bandwidth of 1.875 GHz (divided by 3840 channels resulting in a channel width
of 488.281 kHz). The CO(3$-$2) line was the only spectral line excluded during
the process of the continuum construction. We obtained an image rms noise for
the continuum at 0.8 mm of 10 $\mu$Jy beam-1 at an angular resolution of
$0\hbox to0.0pt{.\hss}{{}^{\prime\prime}}88\times 0\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}76$; 14∘. This angular resolution is very
similar to that obtained in the VLA image (see caption of Figure 1). We used a
robust parameter equal to 0.5 in the TCLEAN task, an adequate compromise
between angular resolution and sensitivity. The parameters of the sub-
millimeter source detected are given in Table 1. The ALMA observations were
not configured to detect any polarized emission. For both the VLA and the ALMA
observations we fitted the data in the image plane with the task IMFIT of
CASA. The results of such fittings are given in Table 1. Our derived value for
ALMA, 207$\pm$10 $\mu$Jy is lower than the value of 316$\pm$20 $\mu$Jy
obtained by Faramaz et al. (2021). This difference is probably due to the fact
that Faramaz et al. (2021) included in their analysis data from the Atacama
Compact Array, recovering more extended flux.

-3.0cm-2cm

Table 1: Parameters of the VLA and ALMA observations of the HR 8799 Region

9  Mean Frequency/ Deconvolved Total Flux Positiona Project Epoch Telescope
Bandwidth Dimensions Density RA(J2000)b DEC(J2000)c 12B-188 2012.88 VLA 3/2
GHz 0$\hbox to0.0pt{.\hss}{{}^{\prime\prime}}$5$\pm$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$2$\times$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$2$\pm$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$2;$+71^{\circ}$$\pm 35$∘ 19.1$\pm$2.7
$\mu$Jy 28$\hbox to0.0pt{.\hss}^{s}$763$\pm$0$\hbox to0.0pt{.\hss}^{s}$003
04$\hbox to0.0pt{.\hss}{{}^{\prime\prime}}$71$\pm$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$03 2016.1.00907.S 2018.38 ALMA 340/8 GHz
0$\hbox to0.0pt{.\hss}{{}^{\prime\prime}}$5$\pm$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$1$\times$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$5$\pm$0.”1;$+160^{\circ}$$\pm$$91^{\circ}$
207$\pm$10 $\mu$Jy $28\hbox to0.0pt{.\hss}^{s}764$$\pm$$0\hbox
to0.0pt{.\hss}^{s}003$ 04$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$68$\pm$0$\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}$05 aFor the epoch of the observations.
b$23^{h}07^{m}$. c$+21^{\circ}08^{\prime}$.

Figure 1: Objects in the vicinity of HR 8799. The contours show the Very Large
Array source observed at 3.0 GHz for epoch 2012.88. Contours are -4, 4, 6, 8
and 10 times 1.28 $\mu$Jy beam-1, the rms noise in this region of the image.
The synthesized beam ($0\hbox to0.0pt{.\hss}{"}71\times 0\hbox
to0.0pt{.\hss}{"}71;-85\hbox to0.0pt{.\hss}^{\circ}6$) is shown in the bottom
left corner of the image. The positions of HR 8799 (star) and its four
exoplanets b, c, d, and e (crosses) have been corrected for proper motions and
the orbital motions of the exoplanets to the epoch of the VLA observations
(2012.88). The large circle marks the inner edge of the debris disk and shows
that all four exoplanets as wel as the VLA source fall inside this inner edge.
The arrow gives the proper motions of HR 8799 between the epoch of the VLA
observations (star) and that of the ALMA observations (2018.38), with the
position of HR 8799 for this epoch indicated with a square.

##  0.3 Interpretation

###  0.3.1 VLA data

The final image (Figure 1) reveals the presence of a 3.0 GHz source about
$2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ to the north of HR 8799. This
radio source does not coincide in position with the star or with any of its
exoplanets and appears to be located at the inner edge of the debris disk of
HR 8799. In Figure 1 we show the radio continuum source as well as the
positions of HR 8799 and its four exoplanets for epoch 2012.88, corrected for
the proper motions of the star from Gaia DR3 (Gaia collaboration et al. 2023)
and an interpolation of the orbital motions of the exoplanets with respect to
the star taken from Konopacky et al. (2016). In Figure 1 we also show the
approximate position of the inner radius of the debris disk. The best-fit
model for this parameter given by Faramaz et al. (2021) is 135$\pm$4 au, that
at the distance of 40.85 pc derived from the Gaia DR3 parallax equals 3.3 +-
0.1.

The parameters of the radio source are given in Table 1. No circular
polarization was detected at an absolute upper limit of 19%. What is the a
priori probability of finding a 3.0 GHz source with a flux density of 19.1
$\mu$Jy in a circle with radius of $2\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}2$? To estimate this probability we have used
the 3.0 GHz source catalog of Smolčić et al. (2017) to produce a plot of the
expected number of background sources as a function of flux density (Figure
2). From this figure we find that the expected number of sources with a flux
density equal or larger than 19.1 $\mu$Jy is 1.1 per square arcmin. Since a
circle with radius of $2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ has a solid
angle of 0.0042 square arcmin, the a priori probability is 0.0046, or 1 in
215. This probability is small, but not stringently improbable.

Another characteristic that may help understand the nature of the radio source
is its spectral index. Dividing the data in two windows of 1 GHz wide each
centered at 2.5 and 3.5 GHz, we obtain flux densities of 16.5$\pm$4.1 and
25.3$\pm$2.9 $\mu$Jy, respectively. These flux densities give a spectral index
of $\alpha$ = 1.3$\pm$0.9 ($S_{\nu}\propto\nu^{\alpha}$). The uncertainty is
large but the value certainly favors a positive spectral index. This is
somewhat unusual for 3.0 GHz background sources since only $\simeq$11% of them
have spectral indices $\geq$0.4 (Smolčić et al. 2017), the 1-$\sigma$ lower
limit of our estimate. Finally, we searched for time variability determining
the flux density of the source for each of the individual six epochs. We found
that all individual six flux densities coincided to the 1-2$\sigma$ level with
the average value given in Table 1, suggesting no variability in the timescale
of days to one month.

Figure 2: Expected number of 3.0 GHz sources per square arcmin as function of
the flux density lower limit. Derived from Smolčić et al. (2017).

###  0.3.2 ALMA data

In the same way that at 3.0 GHz, we can ask how probable it is that a
background source with 0.49 mJy at 338 GHz falls inside the circle with a
radius of $2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ centered on HR 8799.
From the results of Zavala et al. (2017) we estimate that the number of
background sources with a flux density of 0.49 mJy or higher at 850 $\mu$m is
about 4.9 per square arcmin. Then, the a priori probability that such a source
falls inside the circle with a solid angle of 0.0042 square arcmin is 0.021.
Considering that a large number of protoplanetary disks is known, this
apparent association is not unexpected. Just in Orion, several hundred
protoplanetary disks have been detected (van Terwisga et al. 2022). As in the
case of the VLA data, we searched for time variability determining the flux
density of the source for each of the individual six epochs. We found that all
individual six flux densities coincided to the 1-2$\sigma$ level with the
average value given in Table 1, suggesting no variability in the timescale of
days to one month.

###  0.3.3 Comparison between the VLA and the ALMA data

Our first conclusion is that despite the low probability of finding a
centimeter source so close to HR 8799, the coincidence of positions between
the VLA and ALMA data (separated by 5.5 years) rules out the association of
the radio source with the HR 8799 system. The two positions coincide within
$\leq 0\hbox to0.0pt{.\hss}{{}^{\prime\prime}}03$. HR 8799 has large proper
motions, of order $0\hbox to0.0pt{.\hss}{{}^{\prime\prime}}1$ per year
($\mu_{\alpha}\leavevmode\nobreak\ cos\delta=108.284\pm
0.056\leavevmode\nobreak\ mas\leavevmode\nobreak\
yr^{-1};\mu_{\delta}=-50.040\pm 0.059\leavevmode\nobreak\ masyr{-1}$; Gaia
collaboration et al. 2023). In the time interval between the two data sets, we
expect a total displacement of $\simeq 0\hbox
to0.0pt{.\hss}{{}^{\prime\prime}}6$, twenty times larger than our upper limit.
This rules out a possible true association between the radio source and the HR
8799 system.

The deconvolved dimensions of the VLA and ALMA sources are consistent within
the noise. The spectral index between the 3.0 and 340.0 GHz flux densities is
0.69$\pm$0.03. This spectral index and the lack of proper motions suggest that
the radio source is a background millimeter galaxy.

We also note that the data points of Faramaz et al. (2021) have flux densities
of 316$\pm$20 and 58$\pm$18 $\mu$Jy, at 340 and 230 GHz, respectively. These
two points give a spectral index of 4.0$\pm$0.9. As noted by Faramaz et al.
(2021), this value is too steep to be consistent with the typical millimeter
spectral index expected from debris disks (2.5$\pm$0.4; MacGregor et al. 2016)
and is more consistent with typical values expected for extragalactic dust
emission (3.6$\pm$0.4; Casey 2012). This source is thus likely to be a
background millimeter galaxy. Our 3 GHz flux density (19.1$\pm$2.7 $\mu$Jy)
far exceeds the extrapolation of the mm fit to 3 GHz, and at this frequency
has a spectral index of 1.3$\pm$0.9, suggesting this emission is partially
thick synchrotron emission from the galaxy. Recent reviews on protoplanetary
disks and debris disks are given by Andrews (2020) and Hughes et al. (2018),
respectively.

###  0.3.4 The Arp Effect

These unlikely associations between sources in the plane of the sky can be
called the Arp Effect. Alton Arp (1927-2013) was a distinguished astronomer
that studied interacting and apparently interacting galaxies. In some cases,
he found that two apparently interacting galaxies close in the sky had
different redshifts, leading him to question the cosmological interpretation
of redshifts. Several of these apparently interacting sources are described in
his Atlas of Peculiar Galaxies (Arp 1966). An example of this type of sources
is Stephan’s Quintet, that more properly should be called Stephan’s Quartet
because one of the galaxies is much closer than the other four. Arp’s Effect
can be summarized as follows: if you study a sufficiently large number of
sources, you will find some with unlikely apparent associations. Nevertheless,
the topic deserves more research.

##  0.4 Conclusions

We analyzed VLA observations of the A5/F0 V star HR 8799, detecting a source
at $2\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2$ to the north of HR 8799. The a
priori probability of finding such a source is 0.0042. Despite this low
probability that suggests a true association, the lack of proper motions
obtained when comparing the VLA data with ALMA data taken 5.5 years after
favors the radio source being a background millimeter galaxy unrelated to the
HR 8799 system.

As more protoplanetary and debris disks are studied, other apparent
associations are expected to emerge. Another case is HD 95086, where a
millimeter source was found projected on its debris disk (Zapata et al. 2018).
In this case the lack of proper motions also favors that the source is a
background millimeter galaxy. Another example is the source dubbed the ”Great
Dust Cloud” (Gáspár et al. 2023) apparently associated with Fomalhaut, that
has also been interpreted as a background millimeter galaxy (Kennedy et al.
2023).

###### Acknowledgements.

This research has made use of the NASA Exoplanet Archive, which is operated by
the California Institute of Technology, under contract with the National
Aeronautics and Space Administration under the Exoplanet Exploration Program.
This research has made use of data obtained from or tools provided by the
portal exoplanet.eu of The Extrasolar Planets Encyclopaedia. This work also
made use of data from the European Space Agency (ESA) mission Gaia
(https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and
Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement. L.A.Z. acknowledges
financial support from CONACyT-280775 and UNAM-PAPIIT IN110618, and IN112323
grants, México. L.F.R. acknowledges the financial support of DGAPA (UNAM)
IN105617, IN101418, IN110618 and IN112417 and CONACyT 238631 and 280775-CF
grant 263356.

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