Asteroids Characterization Using Data from Large Surveys: Application to Objects with Romanian Designation

ASTEROIDS CHARACTERIZATION USING DATA FROM LARGE SURVEYS: APPLICATION TO OBJECTS WITH ROMANIAN DESIGNATION

IOANA LUCIA BOACA˘ 1, MARCEL POPESCU1,2 1 Astronomical Institute of Romanian Academy Str. Cutitul de Argint 5, 040557 Bucharest, Romania Email: [email protected] 2 Instituto de Astrof´ısica de Canarias (IAC) C/V´ıa Lactea´ s/n, 38205 La Laguna, Tenerife, Spain

Abstract. The data provided by large surveys has revolutionized the astronomy. Sev- eral observing programs such as Sloan Digital Sky Survey (SDSS), Vista Hemisphere Survey (VISTA) and Wide-field Infrared Survey Explorer (WISE) provide spectro-photometric information for a huge number of celestial objects, including Solar System bodies. In this paper we show how the observations performed by these surveys can be used for the characterization of individual objects. As an application, we consider the asteroids with designation of Romanian origin which were observed by the VISTA-VHS survey and reported in the MOVIS-C catalog. We taxonomically classified the asteroids using their colors, the Euclidean distances between their reflectances and the reflectances of the 24 Bus-DeMeo mean spectra and performing a Monte Carlo simulation. We reach the conclusion that (9494) Donici is a S-type asteroid, (9495) Eminescu is a V-type, (26478) Cristianrosu is part of the S-complex, while (263516) Alexescu is a Sa-, R- or O-type candidate . Key words: asteroids – observations – photometry – spectroscopy.

1. INTRODUCTION

Asteroids are small, rocky bodies that orbit the Sun. Most of them (almost 800 000 objects known ∗) are situated in the Main Belt. This is a region situated between the orbits of Mars and Jupiter, at 2-4 AU from the Sun. Other categories include the Trojans, which are small bodies located in the Lagrangian points of Jupiter orbit, the Mars Crossers and the near-Earth asteroids – objects that come in the terres- trial planet region. The Centaurus-like and the trannsneptunaen objects are orbiting in the giant planets region and in the outer part of the Solar system. The asteroids are the remnants of the planetesimals. Their study provides us information regarding the characteristics of the Solar System at the time the planets formed. They can give us information about the chemical composition of primitive nebula because, unlike planets they haven’t modiﬁed their original chemical compo-

∗https://www.minorplanetcenter.net/iau/mpc.html

Romanian Astron. J. , Vol. 29, No. 1, p. 83–95, Bucharest, 2019 84 Ioana Lucia BOACA,˘ Marcel POPESCU 2 sition (Barucci et al., 2009). From the point of view of practical science the study of asteroids can help us prevent future possible collisions between the Earth and near-Earth objects (Barucci et al., 2009). The knowledge of the composition and physical properties of an asteroid would be of great importance when deciding what method to use in order to destroy a potential impactor (Barucci et al., 2009). Compositional information can be inferred from the use of spectrophometric measurements made in wavelengths ranging from ultraviolet, visible and infrared. At first approach, this observational data allows us to make the classification. The three main groups include the S-complex (silicaceous asteroids having spectra with silicate absorption features at 1 µm and 2 µm), the C-complex (carbonaceous chondrites having spectra with flat slopes and small features) and the X-complex (spectra with a moderate slope and small features) (DeMeo et al., 2015). The end member types include peculiar compositions such as the D-types (cometary like spectra), A-types (olivine rich objects) and V-types (basaltic bodies with igneous nature). One of the main tools for determining the properties of a large number of small bodies of Solar System is the use of the large survey data. Among the most extended sky observational programs is VHS (Vista Hemisphere Survey) which surveys the entire southern hemisphere. VISTA (Visible and Infrared Survey Telescope for As- tronomy) is the largest telescope for surveys that uses a near infrared camera with the broad band filters Y, J, H and Ks covering the wavelength range between 0.8 µm and 2.5 µm. The Solar System Objects observed by VISTA survey are included into three catalogues: MOVIS-D (the catalogue that contains all the detections), MOVIS- M (the catalogue that contains the magnitudes of all the objects) and MOVIS-C (the catalogue that contains the colors for the small bodies) (Popescu et al., 2016). The spectro-photometric classification of these objects is reported by Popescu et al. (2018). The Sloan Digital Sky Survey (SDSS) is a digital photometric and spectro- scopic survey that uses the Sloan Foundation 2.5m Telescope at Apache Point Ob- servatory in order to provide photometry in five bands (u’, g’, r’, i’ and z’) in wavelengths from 0.45 to 0.88. SDSS observed over 100 000 asteroids in visible wavelengths having magnitudes smaller than 22.1, 22.4, 22.1, 21.2, and 20.3 in each filter respectively. Their spectrophometric classification was performed by several groups including DeMeo et al. (2013b). The results were used to derive a compositional map of the small bodies across the Solar System (DeMeo et al., 2013b). The SDSS survey had several phases: SDSS-I: 2000–2005, SDSS-II: 2005– 2008, SDSS III: 2008–2014 and SDSS IV: 2014–2020. The latest available data release is Data Release 15 that comprises the observations made in July 2017. The next Data Release (DR16) will be available online on December 2019. The Wide-field Infrared Survey Explorer (WISE) used an infrared-wavelength 3 Asteroids characterization using data from large surveys 85 astronomical space telescope in order to survey all the sky in 3.4, 4.6, 12 and 22 µm wavelength range bands (named W1,W2,W3,W4). The WISE project was extended to NEOWISE with the aim of achieving observations regarding near-Earth asteroids and comets (Mainzer et al., 2011b). Thermal modeling was performed to the observed objects in order to determine the diameter and albedo of the body (Masiero et al., 2017). An asteroid is provided with a person’s name in recognition of one’s merits. An asteroid is usually named after someone renown for his contribution to science or arts, after some important personalities from history or in honour of someone who discovered asteroids or is an important researcher in the field of astronomy. The purpose of this article is to exemplify various methods of taxonomic classification. The observations obtained over various wavelength intervals are considered in order to constrain the compositional type. As an application for using the data provided by the large surveys we studied the asteroids with Romanian designation reported in MOVIS-C catalog, namely (9494) Donici, (9495) Eminescu, (26478) Cristianrosu, and (263516) Alexescu. The reason for selecting this catalog is that near-infrared interval provides the key information for identifying the compositional properties. The information about their designation is briefly summarized bellow based on the corresponding Minor Planet Circulars. Nicolae Donici (1874–1960) was member of the French Astronomical Society in 1897, member of the Russian Astronomical Society in 1899 and honorary member of the Romanian Academy in 1922. He was director of the “Amiral Urseanu” Obser- vatory in Bucharest. In 1945 he started working at the Observatory of Paris-Meudon (Stavinschi , 2015). Mihai Eminescu (1850–1889) was Romania’s national poet. He was chosen post-mortem member of the Romanian Academy on October 28th 1948. Cristian Rosu (born 1991) won an award at the Intel International Science and Engineering Fair that took place in May 2010 in San Jose, California. He devel- oped a project that consisted in a road accident prevention system. Matei Alexescu (1929-1993) was the director of the ”Amiral Urseanu” Observatory in Bucharest dur- ing 1957 and 1978 and founder of the Astronomical Observatory ”Victor Anestin” in Bacau. Donici (9494) and Eminescu (9495) were discovered in 1971 by C.J. van Houten and I. van Houten-Groeneveld at Palomar; Cristianrosu (26478) was discovered in 2000 by the Lincoln Laboratory Near-Earth Asteroid Research Team and Alexescu (263516) was discovered in 2008 by EURONEAR. The article is organized as follows: in Section 2 we describe the main properties of the studied asteroids, in Section 3 we describe the obtained results (the classification of asteroids based on MOVIS colors, the classification of asteroids with the use of Euclidean distance and the classification of asteroids using Monte Carlo simulation) and in Section 4 we present the future observing opportunities. 86 Ioana Lucia BOACA,˘ Marcel POPESCU 4

2. RETRIEVING INFORMATION FOR THE ASTEROIDS

The spectrophotometric data allows to derive taxonomic classification which provides a first indication of their surface composition. One of the most used taxonomic systems is the Bus-DeMeo taxonomy (DeMeo et al., 2013a). The studied objects are classified into 25 classes, based on their chemical composition. The spectral types of this taxonomy were defined by using the Principal Component Analysis over a set of 371 spectra (DeMeo et al., 2013a). We use the colors obtained from large surveys in order to taxonomically classify the objects. As a first step of our study we retrieved the observed images of these objects from the VISTA Science Archive reported in Popescu et al. (2016), as exemplified in Fig. 1. To validate these detections we downloaded the corresponding images and inspected them. This allows us to avoid the misidentification of asteroids. After having inspected all corresponding images, the colors of these objects were retrieved from MOVIS-C catalog. Although the survey data is also provided as databases of sources (including all their properties) found in the observed fields, the visual inspection of the images is a required step when investigating individual objects. This allows to avoid observational artefacts and misidentifications, which are statistically negligible for the entire survey data but for a single object may affect the conclusions.

Fig. 1 – VISTA images taken in the Y ﬁlter for the asteroids presented in this article. The images correspond to (from left to right): (9495) Eminescu (2011-11-11 03:25:33), (9494) Donici (2010-02-24 07:03:34), (26478) Cristianrosu (2011-10-13 02:37:53) and (263516) Alexescu (2013-10-20 05:28:30).

Asteroid Y − JJ − Ks H − Ks Donici 0.346 ± 0.012 0.418 ± 0.024 0.097 ± 0.025 Eminescu 0.773 ± 0.007 −0.007 ± 0.013 −0.60 ± 0.029 Cristianrosu 0.451 ± 0.061 0.434 ± 0.11 0.641 ± 0.121 Alexescu 0.672 ± 0.078 0.671 ± 0.185 – Table 1. MOVIS-C colors

The Y − J, J − Ks, and H − Ks obtained from MOVIS-C catalog (Popescu et al., 2018) are shown in Table 1. Additionally, we used the SDSS Moving Objects Catalog V 3.0 to retrieve information for (26478) Cristianrosu. 5 Asteroids characterization using data from large surveys 87

Asteroid aprop eprop iprop pV H(mag) Diameter Family Donici 2.193 0.114 3.39 0.304 ± 0.048 14.2 3.819 km − Eminescu 2.185 0.127 3.79 0.272 ± 0.091 14.5 3.259 km 8 Cristianrosu 2.771 0.127 9.08 0.205 ± 0.028 14.1 4.750 km 93 Alexescu 2.464 0.112 6.41 0.148 - 0.339 16.8 3.3 km - 5 km 4 Table 2. Properties of asteroids

In order to ﬁnd more information regarding the asteroids from MOVIS-C catalog we accessed the available online databases via web interfaces. Small Bodies Data Ferret∗ provides the colors reported by different surveys. JPL Small-Body Database Browser† summarize both astrometric information and physical properties. Minor Planet Center Database‡ provides astrometric information. A spectral database can be accessed via M4AST§ (Birlan et al., 2016), (Popescu et al., 2012) . AstDys and Asteroids- Dynamic Site¶ contains orbital elements and proper elements for asteroids. The comparison between the observation of asteroids obtained by VISTA-VHS and those obtained by 2MASS is shown by Popescu et al. (2016). We note that the data obtained by VISTA is four magnitudes deeper the data obtained by 2MASS. The reason for not using the data provided by 2MASS in this paper is due to the large errors and inconsistencies of the 2MASS observations for these objects. Statistically these differences are:

J−H J−H J−Ks µV 2M = −0.045, σV 2M = 0.143, µV 2M = −0.045 J−Ks H−Ks H−Ks σV 2M = 0.147,µV 2M = −0.018,σV 2M = 0.108. Hirayama was the ﬁrst scientist to group asteroids into families based on their orbital proper elements (Hirayama, 1918). We used the proper orbital elements provided by the AstDys-2 website (Milani et al., 2014) for comparing the compositional type with the collisional family that the asteroid belongs to. These values are sum- marised in Table 2. The Main Asteroid belt can be divided into three zones based on the value of the semimajor axis as follows: the Inner Main Belt (2.0 < a < 2.5 A.U.), the Central Main Belt (2.5 < a < 2.82 A.U.) and the Outer Main Belt (2.82 < a < 3.7 A.U.). We estimated the size of an asteroid from the diameter D, the absolute magnitude H and the geometric albedo p. The average albedo of the class was used when the albedo was not available on the WISE database. ∗https://sbnapps.psi.edu/ferret/SimpleSearch/form.action †https://ssd.jpl.nasa.gov/sbdb.cgi ‡http://www.minorplanet.info/PHP/lcdbsummaryquery.php §http://m4ast.imcce.fr ¶http://hamilton.dm.unipi.it/astdys/index.php?pc=0 88 Ioana Lucia BOACA,˘ Marcel POPESCU 6

0.4 3:1 5:2 2:1

0.3

prop 0.2 e 9495 26478

0.1 9494 263516

0.0 2.0 2.5 3.0 3.5 4.0

20 prop i

10 26478 9495 263516 0 9494 2.0 2.5 3.0 3.5 4.0 aprop

Fig. 2 – Proper orbital elements (semimajor axis versus inclination and semimajor axis versus eccen- tricity). The mean motion resonances with Jupiter are shown for identifying the largest kirkwood gaps.

According to the proper elements shown in Fig. 2 (9494) Donici, (9495) Em- inescu and (263516) Alexescu are inner belt asteroids, while (26478) Cristianrosu is a middle belt asteroid (see Fig 2). From the point of view of association with collisional families(Nesvorny et al., 2015), (26478) Cristianrosu is part of the (93) Minerva Family, (263516) Alexescu is part of the (4) Vesta Family, while (9495) Eminescu can be dynamically associated with (8) Flora collisional family.

3. RESULTS

In what follows we will ﬁnd the spectral class of the asteroids with the use of two methods: plotting the colors and calculating the Euclidean distance to each spectral class. To account for the errors we perform a Monte Carlo simulation.

3.1. CLASSIFICATION OF ASTEROIDS BASED ON MOVIS COLORS

The ﬁrst step for our analysis is the classiﬁcation based on color-color plots, as shown in Fig. 3. In order to classify the objects according to their colors we use the method presented in Popescu et al. (2016) and Popescu et al. (2018), where the 7 Asteroids characterization using data from large surveys 89 authors clustered the following groups of asteroids: A = (A,Sa), CX = (B,C − complex,X − complex), KL = (K,L), D,V,S = (Q,S complex).

with err <= 0.03 asteroids with Romanian names

−

Fig. 3 – Taxonomic classiﬁcation of asteroids based on MOVIS colors. The plot of (Y − J) versus (J − Ks) colors. The main taxonomic classes are shown.

Plotting the (Y − J) vs (J − Ks) of objects having color errors less than 0.033 clearly shows the separation between the C-complex and the S-complex. A linear ﬁt to the asteroids situated close to the S/C division resulted in equation

±0.046 ±0.016 α(Y −J) = 0.412 · (J − Ks) + 0.155 (1) that marks the separation between the S types and the C and D types. We follow the algorithm from Popescu et al. (2018) and obtain the classification of asteroids as follows: 1) The region (Y − J) ≥ 0.5 and (J − Ks) ≤ 0.3 is representative for the V - type asteroids. 2) The O region is delimited by (Y − J) ≥ 0.5 and 0.7 ≥ (J − Ks) ≥ 0.3 3) The data below the α line are classified into three regions according to the following criteria: the region (Y − J) ≥ 0.3 contains the D types, while the region (Y − J) ≤ 0.22 and (J − Ks) ≤ 0.34 corresponds to the B types. The C/X complex 90 Ioana Lucia BOACA,˘ Marcel POPESCU 8 is situated between the D and B complex (Popescu et al., 2018). We plot the studied asteroids with the corresponding errorbars and the rest of the MOVIS asteroids having errors smaller than 0.03 in the background. Thus, we found that (9495) Eminescu is in the V-type region; (263516) Alexescu, although having large error bars, is associated with the O-types (although Sa and R-types cannot be excluded); (9494) Donici and (26478) Cristianrosu are part of the S complex. In (Mainzer et al., 2011a) the authors determined the mean values of the Bus- DeMeo taxonomic classes as follows: pV = 0.339 (for the O-complex), pV = 0.211 (for the C-complex) and pV = 0.362 (for the V-types). The albedo for (9494) Donici (from Table 2) is smaller than the mean albedo from (Mainzer et al., 2011a), while the albedos for the rest of the asteroids presented in Table 2 are in good agreement with the taxonomic classification resulting from (Mainzer et al., 2011a). These facts confirm the taxonomic classification resulting from Fig. 3.

3.2. CLASSIFICATION OF ASTEROIDS WITH THE USE OF THE EUCLIDEAN DISTANCE

The characterization of these asteroids can be improved with the help of ad- ditional data from other surveys. Several methods can be used to account for these information. Among them, the Euclidean distance provides a simple means of classification in any taxonomic system. The second method used for determining the taxonomic class is that of calculating the Euclidean distances between the reflectances of the studied asteroid calculated in the available VISTA and SDSS filters and the photometric reflectances of all Bus-DeMeo typical spectra. In Table 1 we mentioned the colors of the asteroids in VISTA filters. The SDSS survey provides two sets of colors for (26478) Cristianrosu. We calculated a median value for the colors and the corresponding errors by error propagation. We obtained that (u − g) = 1.75 ± 0.12, (g − r) = 0.62 ± 0.02, (r − i) = 0.21 ± 0.02, (r − z) = 0.21±0.04 and (i − z) = 0±0.04. To account for the colors of the Sun we used (u − g) = 1.40, (g − r) = 0.45, (r − i) = 0.12, (r − z) = 0.16, (i − z) = 0.04 (Holmberg et al., 2006), and (Y − J) = 0.1960, (J − H) = 0.2550, and (H − K) = 0.0820 (Popescu et al., 2018). These are removed because we compute the Euclidian distance relative to the standard reflectance types. In order to taxonomically classify each asteroid, we computed the Euclidean distances between the reflectances computed in each filter and the reflectances of the 25 Bus-DeMeo templates. We retain the minimum of the distances and thus obtain the corresponding spectral class. We present the values of the Euclidean distances in Table 3. From the results presented in Table 3 we reach the conclusion that (26478) Cris- tianrosu belongs to S-complex (and most likely is a Sq candidate), (9494) Donici is a S-type asteroid, (9495) Eminescu is a V-type asteroid. We cannot clearly determine 9 Asteroids characterization using data from large surveys 91

Asteroid First Match Second Match Third Match Cristianrosu Sq S R 0.2681 0.2960 0.3021 Donici S Sr K 0.0313 0.0601 0.0666 Eminescu V Sv O 0.0949 0.3976 0.4369 Alexescu Sa D R 0.2399 0.2534 0.2710 Table 3. Euclidean distance between the colors of the studied asteroids and the standard Bus-DeMeo taxonomic classes

(263516) Alexescu (26478) Cristianrosu 1.6 1.6 O spec 1.4 1.4

1.2 1.2

1.0 1.0

0.8 0.8 Sq spec 0.6 0.6 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 (9494) Donici (9495) Eminescu 1.10 1.0 1.05

1.00 0.8 0.95

0.90 0.6 0.85 S spec V spec

1.0 1.5 2.0 2.5 1.0 1.5 2.0 2.5

Fig. 4 – Reflectances of the 4 asteroids compared to reflectance of the class corresponding to the Eu- clidean distance. the taxonomic class for (263516) Alexescu because we have only two colors available (Y − J) and (J − Ks) with large errors and the O, Sa and R spectral classes have few members. In Fig. 4 we plot the values of the colors of an asteroid in the available filters and the values of the corresponding spectral type calculated in the filter. 92 Ioana Lucia BOACA,˘ Marcel POPESCU 10

3.3. CLASSIFICATION OF ASTEROIDS USING MONTE CARLO SIMULATION

The errors are of great importance compared to the spectra when working with spectrofotometric data. In order to account for the propagation of errors in the spectral results we will perform a Monte Carlo simulation. For each available filter we generated 1E6 random numbers in the interval Rf − errRf ,Rf + errRf with standard uniform distribution, where Rf is the value of the reflectance in the filter and errRf is the corresponding error.

Asteroid Class Probability Donici S 0.8714 Eminescu V 1 Cristianrosu Sv 0.0682 Alexescu R 0.4529 Table 4. Monte Carlo results

For each of the 371 Bus-DeMeo spectra we transformed the reflectance, the colors and the color dispersion. We calculated the mean value of the colors for all spectral classes. For each mean we determined the fluxes and the corresponding errors. The reflectances were normalized relative to the J filter and to the g filter; thus we took RJ = 1 (for the VISTA survey) and Rg = 1 (for the SDSS survey). We calculated the reflectance in each filter for each Bus-DeMeo mean template and the Euclidean distances between the random generated numbers and the values of all templates calculated in the filters. The probability for the asteroid of falling in each spectral class was obtained (by calculating how many times the minimum Euclidean distance is reached for each class and divided it by 1E6) and retained the maximum probability. In Table 4 we emphasize the taxonomic classes with the highest proba- bilities obtained for each asteroid.

4. OBSERVING OPPORTUNITIES

Follow-up observations of these asteroids can provide spectral data, light curves and polarimetric information. The best time to observe these celestial bodies is determined by their apparent magnitude and by the observing geometry. The apparent magnitude variations for all the four objects studied in this paper are shown in Fig. 5. Two magnitude limits are shown with respect to small class telescopes such as those belonging to AIRA (Gherase et al., 2017). A magnitude brighter than 16.5 allows photometric observations with telescopes with apertures in the range of 15-40 cm. From a light-polluted area such, as Bucharest, the detection limit of these telescopes is in the range of 18. 11 Asteroids characterization using data from large surveys 93

(9494) Donici (26478) Cristianrosu (9495) Eminescu (263517) Alexescu

19 magnitude

2019-01-012020-01-012021-01-012022-01-012023-01-012024-01-012025-01-012026-01-01

Fig. 5 – Variation of apparent magnitude over time: Ox axis - time; Oy axis - apparent magnitude

5. CONCLUSIONS AND FUTURE WORK

In this paper we analyse several methods for characterizing asteroids using the data provided by large observing programs. These were applied to the asteroids with Romanian designations as a proof of concept. The databases available online were used (Small Bodies data Ferret, JPL Small- Body Database Browser, Minor Planet Center Database, M4AST, AstDys and Aster- oids - Dynamic Site) in order to ﬁnd information regarding the main properties of asteroids needed in order to characterize the asteroids. The spectral Bus-DeMeo class of the studied asteroids was determined by using three methods: plotting the colors, calculating the Euclidean distance to each spectral class and performing a Monte Carlo simulation. We obtained that (9494) Donici is a S-type, (9495) Eminescu is a V-type, (26478) Cristianrosu belongs to the S-complex and (263516) Alexescu is an O-type. (9495) Eminescu is part of the Flora family (Nesvorny et al., 2015) that belongs to the S taxonomic type. However, the taxonomic type indicates that it is a fugitive from (4) Vesta family. (26478) Cristianrosu is part of the Geﬁon family that has the S taxonomic type (Nesvorny et al., 2015). This result is in agreement to the taxonomic class found by us in this article. (263516) Alexescu belongs to the Vesta family that has a typical V taxonomic type (Nesvorny et al., 2015). However, we found it to be a Sa-, R- or O-type candidate. 94 Ioana Lucia BOACA,˘ Marcel POPESCU 12

Acknowledgements. The work of ILB and MP was supported by a grant of the Romanian National Authority for Scientiﬁc Research - UEFISCDI, project number PN-III-P1-1.2-PCCDI- 2017-0371.

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Received on 22 July 2019