Lulin Observatory Research Articles

Airglow observed by a full-band imager together with multi-instruments in Taiwan during nighttime of 1 November 2021

This study demonstrates an innovative approach of using a full-band chromatic all-sky imager, routinely operational for monitoring sky conditions at Lulin observatory (23.5°N, 120.9°E, 12.5° N magnetic latitude), Taiwan, to investigate equatorial plasma bubbles (EPBs). Distinct north–south aligned EPB depletions are identified by decomposing the color-scale images to respective red (centered at 630 nm) and green (520 nm) channels, where blue channel (470 nm) helps for background suppression. The intense EPBs, drifting eastwards at 60–100 m/s velocity, are also associated with reduced total electron content (TEC) values; increased ROTI (rate of TEC index); remarkable range spread-F; and prominent fluctuations in Doppler frequency shifts as well as FORMOSAT-7/COSMIC-2 electron density/S4 scintillation profiles. The results show that a full-band chromatic imager offers a cost-effective alternative to investigate the EPBs usually detected in OI 630.0 and 557.7 nm airglow emissions.

Long-term monitoring of comet 29P/Schwassmann–Wachmann from the Lulin Observatory

Abstract Multi-band photometric monitoring of comet 29P/Schwassmann–Wachmann 1 was conducted with the standard Johnson–Cousins filter set. Observations extended from 2018 July to 2021 December. The comet was detected to show at least 12 relatively large outbursts, during which its brightness increased by 1.5 to 5 mag as measured through a 5″ aperture. The outbursts resulted in a clear variation of the cometary brightness profile. The derived slopes of the surface brightness profiles showed a significant variation with time from a shallower slope to a steeper one at the beginning of each outburst and then slowly returning to pre-outburst values. There did not seem to be any obvious change in the color indices as the outbursts occurred. However, for the quadruple outbursts in late September of 2021, we could confidently spot a change in the color of the comet, indicating that the color indices seem to be less than the mean values, especially in the B − V term. Dust production derived by using the dust production rate parameter, Afρ, from the R-band photometry measurements shows the outburst to be accompanied by a large increasing trend. Using a simple model and the derived outflow velocity of 0.11 km s−1 from the expanding shell features, an estimated lower limit of 1.0 × 108 kg up to 2.7 × 109 kg of dust was released during the quadruple outbursts by using a specific dust size of 1 μm.

Polarimetric observations of asteroids of different taxonomic classes from Lulin Observatory in Taiwan

Polarimetric measurements are a powerful tool in studying compositions and structures of dust layers of asteroids and other solar system objects. A pilot project was carried out using the Triple Range Imager and Polarimeter (TRIPOL) on Lulin 1-m (LOT) telescope at Lulin Observatory to obtain instrument characteristics essential to polarimetric diagnostics of asteroids. A comparison of the TRIPOL results with previous work from the measurements of a number of standard unpolarized, polarized stars, and 29 main-belt asteroids with known taxonomic types (B-, C-, S-, and M-type) shows that a long-term program of polarimetric observations of asteroids has a promising prospect at Lulin Observatory.

A study of photometric observations of (3200) phaethon from Lulin Observatory

The near-Earth asteroid (3200) Phaethon is regarded as the parent body of the Geminid meteor stream. It is also an active asteroid with a very blue spectrum which is similar to that of the members of the Pallas family. For obtaining this asteroid's surface property in more detail, we conducted a series of multicolor photometry of this asteroid over a wide range of solar phase angle during its close approach to the Earth in the end of 2017. Our observations confirmed that Phaethon is a C-complex asteroid as previous studies have claimed. We also obtained the phase curve for this asteroid in solar phase angle of 22.67 ∼ 114.48°. From this result, we estimate that the absolute magnitude (H) of this asteroid is 14.450 ± 0.017 mag, and its phase slope is (β) ∼0.0422 ± 0.0005. Using the absolute magnitude that we estimated, together with an estimate of Phaethon's albedo (0.122 ± 0.03) that previous literature derived, we can derived the Phaethon's diameter, 4.90−0.54+0.79 km. The diameter estimate is roughly consistent with what was observed through the radar ranging measurement conducted as the same period as our observation.

A study of the physical properties of an active asteroid (6478 Gault)

Abstract In 2019 January, the appearance of asteroid 6478 Gault immediately attracted attention because this object exhibited a long and thin tail that was quite different from the usual asteroids. This unexpected morphology placed asteroid 6478 Gault into the catalogue of active asteroids. We acquired photometric and spectroscopic observations on 37 nights from 2019 January to April using several telescopes, including LOT (1 m telescope) and SLT (40 cm telescope) at Lulin Observatory, and the 2.4 m telescope at Lijiang station of Yunnan Observatory. We did not find any reliable value for the rotational period of Gault during 2.5 hr and 5 hr observations on 2019 January 26 at Lijiang station and March 25 at Lulin Observatory, respectively. We classified 6478 Gault as a Q-type asteroid using visible spectrum and photometric measurements, including colors (B − Vavg = 0.764 ± 0.045, V − Ravg = 0.450 ± 0.023), and relative reflectance. By using Finson–Probstein analysis, the grain size for Gault’s tail 2 is larger than 20 μm.

The Asteroid Rotation Period Survey Using the China Near-Earth Object Survey Telescope (CNEOST)

Abstract We initiated the bilateral collaboration between the Lulin Observatory and the Purple Mountain Observatory to collect asteroid lightcurves using the Chinese Near-Earth Object Survey Telescope at the Xuyi Observation Station. The primary goal of this collaboration was to discover super-fast rotators (SFRs) and study their physical properties. Two campaigns have been conducted: (a) a survey of ∼45 deg2 using 8 minute cadence during 2017 February 26–March 2, and (b) a survey of ∼60 deg2 using 10 minute cadence during 2018 March 9–12. Our samples are mainly main-belt asteroids and some Hildas and Jupiter Trojans. Out of 4522 collected lightcurves, 506 reliable rotation periods were obtained. Among the reliable rotation periods, we found 16 candidates with a possible rotation period of <2.2 hr, in which (134291) 2006 DZ6 shows a very convincing folded lightcurve and the other 15 candidates only have a likely trend. Further confirmation is needed for the rotation periods of these SFR candidates. In addition, (2280) Kunikov seems to have an eclipsing feature on its lightcurve with a relatively long rotation period suggesting that it is likely a fully synchronized binary asteroid. When the preliminary spin-rate distributions were carried out for asteroids using different sizes, no obvious difference was found.

Long-term activity of Comet C/2007 N3 (Lulin) as monitored by the SLT at the Lulin Observatory

Abstract The green comet C/2007 N3 (Lulin) is a new Oort cloud comet that has a retrograde orbit (inclination of $178^{\circ }$). It reached its perihelion on 2009 January 10, and its closest distance to Earth was 0.411 astronomical units (au) on February 24. Soon after its discovery on 2007 July 11, the coma activity of Comet Lulin was monitored closely by an Super Light Telescope 41 cm telescope until 2009 April. After long-term monitoring of Comet Lulin, the dust production rate [A(θ)fρ] was estimated. An unexpected increase in the ${A(0)f\rho}$ near the perigee appears to indicate an opposition effect. By investigating the surface brightness profiles, dust-to-gas ratios, and magnitudes, we ruled out the influences of gas and ion contamination and the outburst phenomenon. We discovered the anti-tail in late December 2008 but were unsure of the composition. We found that this abnormal tail lasted for a considerable time because of the effect of the orbital geometry. We also found that the jet activity coincided with the peak ${A(\theta)f\rho}$ values, and this clue helped us realize what was happening in the dust coma of Comet Lulin.

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.

2012 TC4 - An unusual fast-rotating PHA with C-type taxonomy

The potentially hazardous asteroid 2012 TC4 made an Earth close approach at a distance of 0.11 lunar distance on 12 October 2017. Photometric observations were carried out between October 11 and October 12 at the Lulin Observatory at short cadence. Data analysis of the light curves shows that this PHA has a fast rotation period of 12.25 min, a light curve variation amplitude of 1.2±0.1 mag corresponding to an axial ratio of about 3.0. Our BVRI photometry also indicates that 2012 TC4 is a C-type asteroid. With the C-type albedo of 0.065 and an absolute magnitude of H = 26.613± 0.350, its effective diameter can be derived to be 24.8−8.0+19.1 m.

Photometric survey and taxonomic identifications of 92 near-Earth asteroids

A photometric survey of near-Earth asteroids (NEAs) was conducted from 2012 through 2014 at Lulin Observatory, Taiwan. The measurements of the color indices, B-V, V-R, and V-I allow the classification of 92 NEAs into seven taxonomic types. Of these samples, 39 of them are new classifications. The fractional abundances of these taxonomic complexes are: A ∼3%, C∼6.5%, D∼8%, Q∼26%, S∼37%, V∼6.5%, and X∼13%. This result is similar to that of Thomas et al. (2011) even though the populations of the D- and X-complex with low albedos are under-represented. The ratio of the C-cluster to the total population of S + C clusters are 0.22 ± 0.06 for H ≤ 17.0 and 0.31 ± 0.06 for H > 17.0, indicating a slightly higher fraction of dark-object population with sizes smaller than 1 km.

DISCOVERY OF AN X-RAY-EMITTING CONTACT BINARY SYSTEM 2MASS J11201034−2201340

ABSTRACT We report the detection of orbital modulation, a model solution, and the X-ray properties of a newly discovered contact binary, Two Micron All Sky Survey (2MASS) J11201034−2201340. We serendipitously found this X-ray point source outside the error ellipse when searching for possible X-ray counterparts of γ-ray millisecond pulsars among the unidentified objects detected by the Fermi Gamma-ray Space Telescope. The optical counterpart of the X-ray source (unrelated to the γ-ray source) was then identified using archival databases. The long-term Catalina Real-Time Transient Survey detected a precise signal with a period of P = 0.28876208 ( 56 ) days. A follow-up observation made by the Super Light Telescope of Lulin Observatory revealed the binary nature of the object. Utilizing archived photometric data of multi-band surveys, we construct the spectral energy distribution (SED), which is well fit by a K2V spectral template. The fitting result of the orbital profile using the Wilson–Devinney code suggests that 2MASS J11201034-2201340 is a short-period A-type contact binary and the more massive component has a cool spot. The X-ray emission was first noted in observations made by Swift, and then further confirmed and characterized by an XMM-Newton observation. The X-ray spectrum can be described by a power law or thermal Bremsstrahlung. Unfortunately, we could not observe significant X-ray orbital modulation. Finally, according to the SED, this system is estimated to be 690 pc from Earth with a calculated X-ray intensity of ( 0.7 − 1.5 ) × 10 30 erg s−1, which is in the expected range of an X-ray emitting contact binary.

Ground-BasedBVRITime-Series Follow-Up Observations for the RR Lyrae stars inKeplerField

Time series observations for the 41 RR Lyrae stars in Kepler’s fields were carried out in 2010 to 2013 using a number of meter class (or smaller) telescopes. These telescopes include the 1-m and 41-cm telescopes of Lulin Observatory (LOT and SLT respectively, Taiwan), the 81-cm telescope of Tenagra-II Observatory (TNG, Arizona, USA), the 1-m telescope at the Mt. Lemmon Optical Astronomy Observatory (LOAO, Arizona, USA), the 1.8-m and 15-cm telescopes at the Bohyunsan Optical Astronomy Observatory (BOAO, Korea), and the 61-cm telescope at the Sobaeksan Optical Astronomy Observatory (SOAO, Korea). All of these telescopes were equipped with commercial available CCD imagers, and the observations were done in standard BVRI filters. Photometric calibration of the RR Lyrae light curves was done with standard stars listed in Landolt standard stars [1]. Observations of selected Landolt standard stars (centered on SA 107-456 & SA 110-232) in Johnson-Kron-Cousins BVRI filters, spanning three distinct airmasses, were done with the 81-cm Tenagra II telescope on 25 June 2011. Raw imaging data were reduced with IRAF in the same manner as in the case of the RR Lyrae, and astrometric calibrated with astrometry.net [2]. We calibrated BVRI magnitudes for 40 RR Lyrae stars.

CCD photometry of distant active comets 228P/LINEAR, C/2006 S3 (LONEOS) and 29P/Schwassmann–Wachmann 1

We presentphotometric investigations of three distant active comets, 228P/LINEAR, C/2006 S3 Lowell Observatory Near-Earth-Object Search (LONEOS) and 29P/Schwassmann-Wachmann 1. The data were obtained with the 1-m optical telescope at Lulin Observatory in Taiwan on 2011 February 5 and 6. These comets were observed at heliocentric distances larger than 3 au, all of them appeared to be active. By cometary morphological and photometric studies, the upper limits of the nuclei radii were derived. Also, the surface brightness profiles, Af rho parameters, mass production rates and the coma colours were measured. Finally, we discussed possible driver of activity in comets.

Detection of large color variation in the potentially hazardous asteroid (297274) 1996 SK

Low-inclination near-earth asteroid (NEA) (297274) 1996 SK, which is also classified as a potentially hazardous asteroid, has a highly eccentric orbit. It was studied by multi-wavelength photometry within the framework of an NEA color survey at Lulin Observatory. Here, we report the finding of large color variation across the surface of (297274) 1996 SK within one asteroidal rotation period of 4.656 ± 0.122 hours and classify it as an S-type asteroid according to its average colors of B — V = 0.767 ± 0.033, V — R = 0.482 ± 0.021, V — I = 0.801 ± 0.025 and the corresponding relative reflectance spectrum. These results might be indicative of differential space weathering or compositional inhomogeneity in the surface materials.

Ground-based photometry for 42 Kepler-field RR Lyrae stars

AbstractFollow-up (U)BVRI photometric observations have been carried out for 42 RR Lyrae stars in the Kepler field. The new magnitude and color information will complement the available extensive high-precision Kepler photometry and recent spectroscopic results. The photometric observations were made with the following telescopes: 1-m and 41-cm telescopes of Lulin Observatory (Taiwan), 81-cm telescope of Tenagra Observatory (Arizona, USA), 1-m telescope at the Mt. Lemmon Optical Astronomy Observatory (LOAO, Arizona, USA), 1.8-m and 15-cm telescopes at the Bohyunsan Optical Astronomy Observatory (BOAO, Korea) and 61-cm telescope at the Sobaeksan Optical Astronomy Observatory (SOAO, Korea). The observations span from 2010 to 2013, with ~200 to ~600 data points per light curve. Preliminary results of the Korean observations were presented at the 5th KASC workshop in Hungary. In this work, we analyze all observations. These observations permit the construction of full light curves for these RR Lyrae stars and can be used to derive multi-filter Fourier parameters.

THE TAOS PROJECT: RESULTS FROM SEVEN YEARS OF SURVEY DATA

The Taiwanese-American Occultation Survey (TAOS) aims to detect serendipitous occultations of stars by small (about 1 km diameter) objects in the Kuiper Belt and beyond. Such events are very rare (<0.001 events per star per year) and short in duration (about 200 ms), so many stars must be monitored at a high readout cadence. TAOS monitors typically around 500 stars simultaneously at a 5 Hz readout cadence with four telescopes located at Lulin Observatory in central Taiwan. In this paper, we report the results of the search for small Kuiper Belt Objects (KBOs) in seven years of data. No occultation events were found, resulting in a 95% c.l. upper limit on the slope of the faint end of the KBO size distribution of q = 3.34 to 3.82, depending on the surface density at the break in the size distribution at a diameter of about 90 km.

The Characteristics of Magnetic Signals Observed at Lulin ELF Station and Their Association with Earthquakes in Taiwan

Electromagnetic wave data from years 2009 2010 which were recorded by the extremely-low-frequency (ELF) station at Lulin Observatory in Taiwan (120.87°E, 23.46°N; 2862 m) are analyzed to explore the wave properties and their association with seismic and geomagnetic activities. Specifically, the power spectra, the ellipticity and the orientation of the below-10-Hz wave signals are studied. The correlation coefficients of these ELF signals and geomagnetic activity indices Dst and Kp are within -0.2 to 0.2, indicating geomagnetic activities are not likely sources for these emissions. In the frequency versus time (f-t) spectrograms, a few events exhibit a frequency shift between 4 and 6 Hz and the possible association with seismic activities of magnitudes (M) larger than 4 in the vicinity of Taiwan are examined. The subset of earthquakes in Taiwan with M/r2 (r is the epicenter distance in kilometer from the Lulin station) of greater than 6 × 10-4, a total number of 50, show a higher association with ELF wave events showing a frequency down-shift within 7 days prior to the earthquakes and a frequency bounce-back within 7 days after the earthquakes. The epicenters of these earthquakes are all within a distance of 100 km from the Lulin station and with a depth of 50 km, suggesting that the detected ELF frequency-shift wave events maybe are pre-earthquake electromagnetic signals.

Development of fully depleted CCD imager NCUcam-1 and follow-up observations for PS1 sky surveys

AbstractVisible 4-color simultaneous imager is being developed for a new 2-m telescope at Lulin observatory in Taiwan. The main task of this instrument is quick and efficient follow-up observations for large scale surveys, such as PS1 sky surveys. NCUcam-1 is one of unit cameras for this instrument. It is equipped with a new type of CCD chip, fully depleted CCD, and have superb sensitivity at the wavelength of 1 micron. Igneous asteroids exhibit prominent absorption feature around 1 micron due to minerals, and NCUcam-1 is able to deliver reliable taxonomic classifications. NCUcam-1 has achieved the first-light using 1-m telescope at Lulin in July 2011. Results of the characterization work for NCUcam-1 and tests observations of asteroids are reported. Upcoming plan for the study of pair asteroids search and identification is also presented.

Physical studies of 81P/Wild 2 from the last two apparitions

Context. We downloaded data for the comet 81P/Wild 2 for May and June 2004 from the ESO archive and monitored this comet from January to August 2010 during its 6th perihelion passage since its discovery in 1978. Photometric data were used to monitor the comet’s gas and dust activity as a function of heliocentric distance. Non-photometric data were used for an analysis of the evolution of the dust coma morphology.Aims. The goal of the analysis of the observational data was to characterize the evolution of the cometary activity as the nucleus approaches the Sun. We also aimed to assess the gas and dust production rates for the last two apparitions and investigated the evolution of the dust coma morphology during the last passage in 2010.Methods. The long-slit spectra data were acquired with the EFOSC2 instrument mounted at the 3.6. m ESO telescope in Chile, while narrowband and broadband images were obtained using the 1-m telescope at the Lulin Observatory in Taiwan. Image enhancing techniques were used to investigate the evolution of the dust coma morphology in the R-broadband images, and we modeled the evolution of these dust jets. Where possible, we studied the dust and gas production rate, and the radial profiles of the dust brightness in the Sun-anti-sunward directions.Results. The morphological analysis helped us to detect the jet structures in the dust coma by using the Larson-Sekanian filter. In May 2004, only one feature had been found on the sunward side. In 2010, one of the jet features had been switched-off after January, and at least four new jets were found from April to May. The physical properties show an average reddening between 4420 A and 6840 A of 8.4% per 1000 A and the average resulting log of the C2 -to-CN production rate ratio is –0.454 ± 0.13, which would place Wild 2 in the “depleted” category according to the taxonomic classification. The slope of the radial dependence of the gas production rates for CN and C2 is very consistent with observations of many other Jupiter-family comets. The slopes of the surface brightness lie between 0.98 and 1.38 in the images taken at the Lulin observatory, whose the range interval from 2000 km to 30 000 km is roughly consistent with the ρ -1 law. However, the gradient of the radial profile in the sunward direction in the outer region (4.0 ≤ log ρ (km) ≤ 4.5) is slightly steeper than that in the inner region (3.3 ≤ log ρ (km) ≤ 4.0).

Optical emissions and behaviors of the blue starters, blue jets, and gigantic jets observed in the Taiwan transient luminous event ground campaign

[1] On 22 July 2007, 37 blue jets/starters and 1 gigantic jet occurring over a thunderstorm in the Fujian province of China were observed from the Lulin observatory on the central mountain ridge of Taiwan. The majority of the jets were observed to occur in a 5 min window during the mature phase of the jet-producing thunderstorm. These jets have significant red band emissions. However, the blue emissions from these jets were not discernible due to severe atmospheric scattering. A model estimation of the emissions from a streamer reveals that the red emissions in blue starters and blue jets are mainly from the nitrogen first positive band (1PN2). The type II gigantic jet is the first of this type that was observed from the ground. The generation sequence of the gigantic jet begins with a blue starter, then a blue jet occurs at the same cloud top after ∼100 ms and finally develops into a gigantic jet ∼50 ms later. Using “optical strokes” as surrogates of the lightning strokes, the correlations between jets and the cloud lightning are explored. The results indicate that the occurrence of jets can be affected by the preceding local cloud-to-ground (CG) lightning or nearby lightning (intracloud (IC) or CG), while in turn the jets might also affect the ensuing lightning activity.