Research of activity of Main Belt Comets 176P/LINEAR, 238P/Read and 288P/(300163) 2006 VW139

As a new class of comet, main belt comets (MBCs) have attracted more and more attention in recent years. To study activity and physical properties of three MBCs 176P/LINEAR, 238P/Read and 288P/(300163) 2006 VW139, we carried out broadband CCD photometry of three MBCs on UT 2016 November 18–19 with the 1-m optical telescope at Lulin Observatory in Taiwan. By comparing cometary surface brightness profiles to stellar surface brightness profiles, and by comparing cometary absolute magnitude to the expected magnitude of inactive nucleus, we found that 176P/LINEAR was inactive, while 238P/Read and 288P/(300163) 2006 VW139 were active. By photometric studies, we obtained the Afρ values and the dust production rates. Finally, the activity of three MBCs were discussed. Our photometric results show that the total dust mass of 238P/Read and 288P/(300163) 2006 VW139 obtained in this work are of the same magnitude as the majority of known MBCs.

www.nature.com/scientificreports www.nature.com/scientificreports/ the activity observed in 2005 19 . The last perihelion passage of 176P was on 2017 March 12. Hsieh et al. 20 examined the pole orientation and active region of 176P and suggested that the comet was active due to a seasonal variation of the solar flux at the active area.
238P/Read (formerly P/2005 U1, hereafter 238P) was discovered by M. T. Read using the Spacewatch 36 inch telescope on Kitt Peak on 2005 October 24. It was the second MBC to be discovered. When it was discovered, it showed cometary activity. 238P repeated activity during its 2011 and 2016 perihelion passage [21][22][23] . The last perihelion passage of 238P was on 2016 October 22.
288P/(300163) 2006 VW 139 (also known as asteroid 300163, formerly 2006 VW 139 , hereafter 288P) was discovered in 2006 and first observed to be active on UT 2011 August 30 24 . 288P was reported the reactivation during its 2016 perihelion passage 23,25 . The last perihelion passage of 288P was on 2016 November 08. Agarwal et al. 26 found that 288P is a binary main belt comet which ejected dust grains via ice sublimation and they suggested sublimation torques may play an important part in binary orbit evolution.
In this paper, we present optical observations and the surface brightness profile (SBP) of the above three MBCs observed on November 18-19, 2016. We also obtained the Afρ values and the dust mass production rates. The activity of three MBCs was discussed. Hsieh et al. 23 published a paper about the 2016 reactivations of 238P and 288P recently, they reported observations of 238P and 288P from 2016 July to 2017 January in this paper. Our observation dates are in this time frame, but are not included in their observation Logs. Thus, the photometric results of our observations can be used to help fill in gaps of their observations.

Methods
The three comets were observed by using the 1-m optical telescope at Lulin Observatory in Taiwan on 2016 November 18-19. This telescope has been equipped with an Alta U42 2 k × 2 k CCD camera. The pixel scale of camera is 0.348 arcsec, the field of view (FOV) is 11.9 × 11.9 arcmin 2 . The average seeing is 1.2 arcsec during the observations.
The three comets were observed through Asahi broad-band R filters. The effective wavelength of the R filter is λ e = 6578 Å, the full width at halfmaximum (FWHM) is Δλ = 1215 Å. The observation mode of telescope was set to track the sidereal motion, the exposure times of comets were chosen to make the apparent motion of the comet within the seeing disc. The details of observations are provided in Table 1.
All images were reduced and calibrated in similar procedures (bias subtraction, flat-field correction and cosmic ray cleaning) used in our previous work 27 . The bias value used in the calibration was an average of several zero-exposure images. The final flat-fields were obtained from several images of the twilight sky. The night sky level used for photometry in the IRAF task PHOT was obtained from the region far from the nucleus. The NOMAD1 catalog was used to perform the magnitude calibration of the images. To minimize the effect of color terms, we selected Standard stars that optical colors were similar to the Sun.

Results
Cometary activity and surface brightness profile. All three MBCs looked like a stellar appearance in each single exposure frame. To increase the signal-to-noise ratios of both our target comets and field stars, we created two composite images per object, one combining all R-band images of each object aligned on the comet, and another combining all images of each object aligned on field stars. The combined frames still appear stellar (Fig. 1). To search possible the extent of coma, we extracted surface brightness profiles (SBPs) of comets and stars from the combined image using the method described in Shi & Ma 28 . By comparing with the stellar SBP, we find that 176P's SBP is consistent with stellar SBP, 238P and 288P's SBPs show a flux excess in outer region. This means that 176P was inactive or unresolved activity on November 19, 2016, while 238P and 288P were active on November 18, 2016 and November 19, 2016, respectively.
The cometary dust production and dust mass production rate. The cometary dust production is usually made by means of the parameter Afρ value (cm) 29 . It is the product of the average grain albedo, the filling factor f (the ratio of the cross section of the dust grains to the field of view of aperture) and the projected radius of the photometric aperture ρ. Afρ can be given by the formalism where R h is in AU, Δ and ρ are in cm, m comet is the comet integrated magnitude. For a steady state coma, Afρ should be an aperture-independent parameter, this parameter can be used to compare measurements concerning the dust continuum produced under different observing conditions, times and instruments.  www.nature.com/scientificreports www.nature.com/scientificreports/ The dust mass loss rate can be calculated by dividing the total dust mass by the time of residence of the dust grains as they travel across the projected photometry annulus 30 . The coma magnitude between φ 1 and φ 2 is given by , where σ is bulk density, a dust is average grain radii, C d is the total cross-section of the coma dust particles and can be computed from Equation (1). The time of residence of the dust grains in the annulus between 1 and φ 2 are expressed in radians,v ej is the radial outflow speed of the dust grains from the nucleus in m s −1 . For comet Hale-Bopp, expansion measurements showed that the radial outflow speed of gas from the nucleus was where v 0 = 550 ms −1 and r 0 = 5 au. For spherical grains emitted from a homogeneous nucleus, a dust velocity is about 10 per cent of the gas velocity 32,33 . So the dust grain ejection velocity is v ej = v(R h )/10. We adopted the dust grain radius value of a dust = 10 μm, this value consistents with dust modeling results for 133P 34 . The grain density was adpoted the canonical asteroid density σ = 2000 kg m −3 35 . For 238P, m 1 = 20.47 ± 0.06, m 2 = 20.29 ± 0.06, the calculated dust production rate is 1.9 kg s −1 . For 288P, m 1 = 19.75 ± 0.05, m 2 = 19.61 ± 0.05, the calculated dust production rate is 4.2 kg s −1 ( Table 2).

Discussion
For 176P, Hsieh et al. 20 derived best-fit IAU phase function parameters of H = 15.10 ± 0.05 and G = 0.15 ± 0.10. Using these phase function parameters, Hsieh et al. 19 summarized apparent R-band magnitude and absolute R-band magnitude of 176P observed before 2014. To compare previous observation to ours, we computed absolute magnitudes based on the total magnitude using the HG approximation with scattering parameter G = 0.15 ± 0.1 and obtained m R (1, 1, 0) = 14.96 ± 0.22. Considering rotational variations of 176P is about 0.7 mag (a peak-to-trough photometric range) 20 . The absolute magnitude obtained in this work is still in the range of rotational variations, this also suggest that it was inactive on November 19, 2016. Table 3 summarized available R-band photometry results of comet 238P. The inactive photometric behavior of 238P has been previously established by Hsieh, Meech & Pittichová 21 who derived best-fit IAU phase function parameters of H = 19.05 ± 0.05 mag and G = −0.03 ± 0.05. Using G = −0.03 ± 0.05, we can then compute the equivalent absolute magnitudes (at heliocentric and geocentric distances of R h = Δ = 1 au and a solar phase angle of α = 0°) for all observations of 238P (Table 3). Comparing absolute magnitude and Afρ values obtained in this work to previous observation, we can find that there is an obvious about 2 mag photometric enhancement in this work than data obtained in 2010 July and August when 238P was observed to be largely inactive, this also suggest that it was active on November 18, 2016. Table 4 summarized available R-band photometry results of comet 288P. Absolute R-band magnitudes (at R h = Δ = 1 au and α = 0°), were computed by using G = 0.15 ± 0.1 20 ( Table 4).
As the determination of the dust production rate of a comet is highly model-dependent and parameter-dependent, we can't compare the dust production rates obtained in this work to other works directly.   Table 2. Magnitude, Afρ, dust mass production rate and total dust mass measured in R-band of comets 176P/ LINEAR, 238P/Read and 288P/(300163) 2006 VW 139 Note. a The total magnitude in the reference aperture of 1.8 arcsec; b The total magnitude in the reference aperture of 4.0 arcsec; c Afρ value in the reference aperture of 1.8 arcsec; d The dust mass production rate in the reference aperture of 1.8 arcsec; e The total dust mass in the reference aperture of 4.0 arcsec.
www.nature.com/scientificreports www.nature.com/scientificreports/ 28 (when 288P was at ν = 5°.6 Examining previously reported photometry of active dust emission, Hsieh et al. 23 found that activity of 238P in 2016 were lower than the activity in 2010 and the activity of 288P in 2016 were larger than the activity in 2000. Comparing the total dust mass of 238P and 288P obtained in this work to Hsieh et al. 's 23 work, we find that our results are consistent with Hsieh et al. 's 23 conclusions. Comparing the total dust mass of 238P and 288P obtained in this work to other MBCs 37,38 , we find that the total dust mass of 238P and 288P obtained in this work are of the same magnitude as the majority of known MBCs's. This is consistent with the fact that almost all of the MBCs appear to eject nearly identical quantities of dust 37 .

Data Availability
Observations were carried out with the 1-m optical telescope at Lulin Observatory in Taiwan. The observation data can be obtained from Lulin Observatory.   Table 4. Summary of available R-band photometry results of comet 288P/300163. Note. a Is visible activity detected?; b The heliocentric distance in au; c The geocentric distance in au; d The phase angle(Sun-comet-Earth) in degrees; e The true anomaly in degrees; f The total magnitude; g Absolute R-band magnitude.